Sebastian Vettel's Ferrari set the pace in Thursday practice for the Monaco Grand Prix. That was no great surprise for a car/driver combination that has won two of the first five races of the 2017 Formula 1 season. But behind Vettel, almost half a second behind him in fact, the order was highly unusual by F1 standards.

Red Bull was unexpectedly second fastest; the Toro Rosso drivers were an even more remarkable fourth and fifth - and ahead of the second Red Bull; and Sergio Perez's customer-powered Force India was quicker than both works Mercedes drivers, who were also split by Kevin Magnussen's Haas - a lower points contender at best ordinarily.

We are told Monte Carlo should be beleaguered McLaren-Honda's best chance yet to shine, on a tight and twisty circuit that should maximise the strengths of its updated chassis and minimise the weaknesses of its troublesome Honda engine, yet neither of its drivers troubled the top 10 on Thursday.

Q3 regular Felipe Massa was only 13th quickest, and the man he has battled with so often in the top 10 shootout over the first five races of the season - Renault's Nico Hulkenberg - was down in 17th, quicker only than team-mate Jolyon Palmer (who suffered an engine problem) and the two Saubers.

A strange day indeed, but there are some logical explanations for the anomalous chaos wreaked on F1's competitive order on the first day of running ahead of the Monaco Grand Prix.

MERCEDES PICKS THE WRONG PATH

Everything was looking pretty good for F1's championship-leading team after the first free practice session. Lewis Hamilton lapped almost two tenths faster than title rival Vettel and was feeling good, beginning to find that crucial rhythm you need to keep going quickly around this place.

But the second session did not go to plan at all. Hamilton and team-mate Valtteri Bottas both completed underwhelming early runs on the super-soft tyre, fractionally quicker than Kimi Raikkonen's Ferrari but almost a full second slower than Vettel.

Hamilton complained about how difficult it was for his W08 to generate tyre temperature on the super-soft compound, and his lot did not improve much when Mercedes bolted on the ultra-soft tyres. Hamilton and Bottas managed to improve by about half a second on the quickest compound, but that proved only enough to leave them languishing down at the lower end of the top 10.

Pure pace ranking

The fastest outright times from each team in practice

Raikkonen was fractionally slower than Daniel Ricciardo's Red Bull, but carried a half-second deficit to Ferrari team-mate Vettel in FP1 through to the afternoon session as well, so he has clearly not got everything hooked up in his SF70H yet, despite stating that he was generally happy with how his car felt.

But into that yawning half-second chasm between Vettel and the rest, the Mercedes drivers should have stepped. Puzzlingly, they couldn't come close.

Stringing a clean lap together in Monaco is always difficult when the track is so busy, and Vettel could have been another 0.125s faster had he put his best sectors together. Vettel also dropped time on his actual fastest lap by doing his best impression of a rallycross driver coming out of the final corner.

Mercedes did nothing of note on the ultra-soft tyre. Bottas completed three laps in the 1m13.9s, but putting his best sectors together lowers his theoretical best lap time to 1m13.571s - enough to jump Magnussen, Hamilton and Perez, but still shy of the Red Bulls, Toro Rossos and Ferraris.

Hamilton aborted all his flying laps apart from that 1m13.873s best, so apart from one first sector that was 0.123s quicker than his actual best lap there is not much more he could theoretically have done.

"I don't understand it," said Hamilton when asked for an explanation after the session. "FP1 was really good, the car was feeling great. Then there was night and day difference; black and white.

"We couldn't work the tyres for some reason, lots of sliding about. I don't know why the tyres weren't working."

Mercedes team boss Toto Wolff suggested Mercedes took a "wrong junction" in setting up its cars for the second session.

"We couldn't back out of it," he told Sky Sports. "The changes are dramatic and you need a couple of hours to undo it, so we decided to push through and collect some data."

This scenario appears to have echoes of Mercedes' practice struggles at Sochi, another circuit where a combination of smooth asphalt and a lack of long-duration corners makes it difficult to load up the tyres properly and get them into their correct working temperature range. Only this time it seems Mercedes found that range only to lose it again with adjustments to the car between sessions.

Hamilton spoke on Wednesday about how difficult the W08 is to drive and set-up correctly, and it looks as though Mercedes has yet to find a sure way to broaden what most teams - Ferrari aside - are finding to be a much narrower set-up window than before, despite the impressive raft of updates it brought to Spain last time out.

"It is harder [to set-up the 2017 cars]," agrees Force India technical director Andrew Green, who reckons the challenge of getting the Pirelli tyres to work in Monaco is a "step more than Sochi".

"It's not so much the generation of car but the generation of tyre. It is harder to get the tyres into the window and this track being such a low energy track, the energy going into the track is so much lower than any track we go to, trying to get energy into the tyres is incredibly difficult.

"They require a reasonable hit to get into the working window and as soon as they fall out, it becomes very, very difficult to get them back in again. It's magnified with the tyres, because if you don't get the car in the sweet spot from a balance perspective you cannot put the energy in the tyres, your tyres don't run at temperature and then all of a sudden a little change can put you a long way off.

"Maybe that is what Mercedes did - but for sure it hurt us."

Green says Force India also went backwards with its set-up for the second session, just not to such an extent that Sergio Perez and Esteban Ocon weren't able to improve their lap times compared to FP1. Mercedes was the only team to go slower in FP2 than it did in FP1.

Perhaps, as in Sochi, Mercedes is only a few set-up adjustments away from getting the car back to its FP1 best. If not, then Hamilton and Bottas could be in serious trouble on a circuit where single-lap pace counts for everything.

RENAULT ALL AT SEA

It would seem even worse set-up trouble befell the Renault works team. While it's true the outfit lost vital track time in each session thanks to technical problems - electrical trouble on Nico Hulkenberg's car in FP1 and an engine problem that stopped Jolyon Palmer's car out on track in FP2 - the lap times were also nowhere near what we've come to expect from a team that has become a Q3 regular this season.

Just under three tenths of a second covered nine cars from seventh placed Perez to his 15^th placed Force India team-mate Ocon in FP2, but the best Hulkenberg could manage was almost eight tenths further back.

"We need to really investigate what's going on, because I'm sure something is not right," said Hulkenberg, who feels Renault will need to try some "radical" set-up changes to get its weekend back on track.

"It felt really poor out there - not just that I'm struggling with the balance and confidence in the car, but also the grip from the tyres. What we get seems really odd and poor.

"Monaco is very specific, and since last year there's been a lot of resurfacing done on many parts of the circuit - very smooth Tarmac and asphalt. Maybe that also plays a role.

"Straight from first run I felt like I'm not going anywhere. I wasn't even looking at my lap times initially, because I knew they were terrible."

The fate of both Mercedes and Renault in FP2 in Monaco shows how drastically things can go wrong if you don't nail the set-up with cars and tyres that are now substantially different to their predecessors.

TORO ROSSO IN THE ZONE

One team that certainly had no problem finding the sweet spot around Monaco was Toro Rosso. Both its drivers looked comfortable and confident from the early laps of the first session and that continued throughout the day.

Daniil Kvyat was sixth fastest and within seven tenths of the pace in FP1, without using the ultra-soft tyre, and he and team-mate Carlos Sainz Jr were fourth and fifth in FP2, only fractionally slower than Ricciardo and Raikkonen and separated from each other by just 0.069s.

There was a certain degree of surprise at this performance, given how Toro Rosso was not expecting the updated STR12 to be as strong around this track as the team has been here in recent seasons.

"The car we have here is the same one as Barcelona, just obviously set-up adapted to this track, and it seems to be working really well at the moment," said Sainz. "We haven't done any big set-up adjustments."

It's likely the circuit has played into Toro Rosso's hands, given it has historically produced draggy cars that are not penalised so heavily on street circuits like Monaco and Singapore. But it was also apparent that both drivers were confident and really hustling their machines.

"The car has a lot of potential every time we put it in the right window," added Kvyat. "It's a very competitive car, but to put it in the right window we have to do a lot of work and today we clearly did it correctly, unlike in Barcelona.

"The car was very pleasant to drive, very reliable, very predictable for me, very suitable for my driving style, and as a result we are P4 and P5.

"In Monaco on Thursday you should not be too clever. If the car feels right, you just should run and get the confidence, because driver's confidence here is a lot."

Kvyat's Red Bull stablemate Ricciardo echoed this sentiment after a session in which he was the only driver to lap within half a second of Vettel's pace. The RB13 looked good from the off in FP1, both Ricciardo and Verstappen able to lean on it and find a good rhythm.

Verstappen would probably have pipped Ricciardo to the second fastest time had he not come across his team-mate in the final few corners, while Ricciardo could have shaved at least another tenth off his own best had he put all his best sectors together.

"I felt like we are sort of back in the groove around here," Ricciardo said. "There's a few little things that I definitely want to work on, but generally we are in that window where we need to be and not a second off or anything, so that's promising.

"I think especially around a street circuit you are better off driving the car as oppose to turning it upside down. That can take confidence away from you, so I'll do some little tweaks to prepare for Saturday but not too much."

WHY MCLAREN IS NOT YET IN THE MIX

McLaren introduced a significant upgrade to the MCL32 for the previous race in Spain, and star driver Fernando Alonso utilised it to full effect to qualify seventh, best-of-the-rest behind the top three teams.

The race did not go to plan, but Alonso's qualifying performance suggested McLaren could be in very good shape coming to Monaco, where a lack of engine power should hurt it far less than it has so far this season.

Alonso is not in Monaco of course, but his stand-in Jenson Button tried McLaren's fully updated car in the team's simulator before coming here and was encouraged by what he discovered.

Alonso already felt this year's McLaren would be a top five car if it had a better engine in the back of it, before the first raft of updates found its way onto the car in Spain, so theoretically the team should be capable of clearly topping the midfield around a circuit where engine limitations count for far less.

But Button and team-mate Stoffel Vandoorne were outside the top 10 in Thursday practice, and it would seem that a lack of confidence, more than anything else, is what's currently preventing them from unlocking those final few tenths that could elevate the team into points contention.

Vandoorne has been struggling all season to get comfortable in the McLaren, requiring serious discussions with the team ahead of this race to attempt to get things back on track, while Button lapped only 0.035s slower despite not having driven a Formula 1 car since November last year, admitting he found it difficult to adapt to the cornering speeds of this new breed of car once McLaren had bolted on all the best bits for FP2.

McLaren may well have a very fine chassis, one capable of being the fourth best around this circuit in the right hands perhaps, but its pace relative to its midfield rivals on Thursday shows the extra value of having a fully confident driver, totally dialed in to the car underneath him - even allowing for the fact that Honda's remaining deficiencies will still be holding McLaren back to some degree.

Monaco is a confidence track, and this season Alonso has looked more committed and confident in his driving than he has at any time since McLaren-Honda made its F1 comeback. But he is not here, and McLaren is arguably suffering for his absence.

If Alonso were not in Indianapolis this weekend, the current competitive order would probably look a little different. As it would had Mercedes not dropped the ball with its set-up on Thursday afternoon.

Formula 1 always depends on such fine margins, and there is nowhere that punishes such mistakes more heavily than Monaco.