Mercedes on the back foot at F1's circuit of contradictions

Mexico City's Autodromo Hermanos Rodriguez is the ultimate Formula 1 outlier. Thanks to its elevation - 2285 metres above sea level, it offers a unique set of challenges to the teams and represents an environment where precise, finely-tuned 21st century grand prix cars are least in their element.

This also makes it an unpredictable circuit. With air density around 25% lower than at sea level the demands on the engine, aerodynamics and cooling are extreme and it's no forgone conclusion who will have the best compromise.

That's what makes predicting form here trickier than at the average track, with what we saw so far arguably one of the most inconclusive Fridays of the year.

It was no surprise to see Ferrari setting the pace during Friday practice, with Sebastian Vettel outpacing Max Verstappen by 0.115 seconds. But Valtteri Bottas in the lead Mercedes was further behind than you might expect, 0.614s off.

SINGLE-LAP PACE

1 Ferrari (Vettel), 1m16.607s
2 Red Bull (Verstappen), 1m16.722s
3 Mercedes (Bottas), 1m17.221s
4 Toro Rosso (Kvyat), 1m17.747s
5 McLaren (Sainz), 1m18.079s
6 Renault (Hulkenberg), 1m18.261s
7 Racing Point (Stroll), 1m18.362s
8 Alfa Romeo (Raikkonen), 1m18.681s
9 Haas (Grosjean), 1m18.766s
10 Williams (Russell), 1m19.968s

Even less surprising was that the key to Ferrari's pace advantage was the first sector, which comprises the long main straight, the right/left/right shimmy through Turns 1-3 and then another short straight. The best non-Ferrari was just over four tenths down through this sector.

What was unexpected was that it was Verstappen's Red Bull that was best-of-the-rest and just over a tenth quicker than the best Mercedes. Given Verstappen also set the pace through the second sector, which features the sweeping Turn 7-11 esses, and the final sector through the twisty stadium section, that bodes well for Red Bull.

A look at the fastest times for each of the top five drivers is revealing, especially given all of their best sectors - save for Lewis Hamilton, who was almost a tenth-and-a-half off his best in the final sector - contributed to their fastest laps.

This isn't the first time Red Bull has looked encouragingly fast on a Friday and recent history suggests it won't have the single-lap pace to be a threat in qualifying. But it's not impossible that the unusual conditions could put Red Bull in stronger contention on Sunday than it has been in recent times.

"I don't expect to fight for pole tomorrow but we just have to focus that we have a good race car," was Verstappen's summary.

Unfortunately for Red Bull and Mercedes, the indicators, tentative as they are - especially given the predicted rain - point to Ferrari having the advantage in qualifying. With track position difficult to gain, even once settled into a race run where the Ferraris are generally not as strong, it could be very difficult to wrest control even with a pace advantage.

But the unusual altitude of the circuit means there are still some questions to be answered. While the turbocompresser mitigates the disadvantage of the reduced air density that would make a normally-aspirated engine around 25% less powerful, this comes at a cost.

The turbo has to work harder so you end up with less energy to harvest and then redeploy. This means the power unit packages, and the inevitable compromises for this outlier circuit, are being tested in a very different way to before and the balance of using the turbo to make the conventional engie work and maximise electrical power is a little different.

"These are all conflicting requirements. What do you focus on?" Jock Clear

Then there's the car challenge. Teams run in their highest-downforce trim, but it delivers around 60% of the overall downforce thanks to the reduced air density. That equates roughly to Monza levels, which also makes things difficult when it comes to the tyres as the cars visibly slide around more dramatically. This is why this track is, according to Ferrari's Jock Clear, "the biggest compromise of the year" in terms of aero package.

"Long straights, you don't want to be going 10-12km/h slower than your competition or you'll get overtaken at the first corner even if you've got a fast car," says Clear.

"Once you're behind, you can't overtake them. We are going to be doing that [going fast on the straights] because we haven't got much downforce and drag but that's the same for everybody.

"Then the series of low-speed combination corners [Turns 4-6 and 13-16] need balance and grip, that needs more downforce.

"Then there's the high-speed esses where you need stability and response, that's a bit like the first sector at Suzuka. So for the low-speed you have extensive use of kerbs and soft ride, which conflicts.

"High-speed balance and high-speed stability is all about keeping the car a flat platform, but riding kerbs is about having a lot of wheel movement and the chassis quite soggy.

"Then cooling is critical, that conflicts with all of these because opening cooling ducts loses downforce.

"So basically, these are all conflicting requirements, and another thing is, what do you focus on? The team that does that best here, will be the team that makes the right compromise."

The long-run pace from the afternoon is best described as inconclusive. With tyre graining a significant problem and struggles keeping the tyres in the right temperature window, degradation was accelerated significantly.

On the soft Pirellis, Verstappen was the quickest for Red Bull with an advantage of 0.371s over the Mercedes of Bottas. Don't read too much into the pace of the Ferrari, with Leclerc's lap times plummeting thanks to degradation.

LONG-RUN PACE (SOFTS) - 6 laps

1 Red Bull (Verstappen), 1m21.818s
2 Mercedes (Bottas), 1m22.189s
3 Racing Point (Stroll), 1m22.876s
4 Toro Rosso (Kvyat), 1m23.099s
5 Ferrari (Leclerc), 1m23.140s
6 McLaren (Norris), 1m23.246s
7 Renault (Ricciardo), 1m23.560s
8 Haas (Magnussen), 1m24.000s*
9 Williams (Russell), 1m23.702s**
10 Alfa Romeo (Raikkonen), 1m23.776s

*4 laps
**2 laps

Given the degradation on the softs, anyone who can use mediums in Q2 and still make the top 10 will do so. Pirelli anticipates a two-stop race, although some drivers suggested it felt more like a four-stop race based on Friday's running.

Data on the medium rubber was harder to come by, although both Vettel and Hamilton conducted long runs on the mediums and. Comparing eight laps of those runs, Hamilton had a slender advantage of less than a tenth - although Vettel's run went on longer and could go a further four laps, with the gap increasing to almost two-tenths on average.

What is clear is that Mercedes still has some more work to do. Bottas admitted that he was surprised by the gap, even though Mercedes expected to struggle, while Hamilton accepted there should be more to come.

"Our pace in the first session looked OK but we seem to have slipped backwards a bit going into the second session," said trackside engineering director Andrew Shovlin. "We've already got a few ideas as to where we may have taken a wrong turn but it's both single-lap pace and long run degradation that we need to improve.

"We need to get the car sorted before we can start talking about what we can achieve in qualifying and the race - it's all interesting stuff from an engineering point of view."

So who will have made the best compromise in Mexico? That remains to be seen, although given Ferrari looks again to have the single-lap pace to get in front on Saturday it might have the tools to achieve the one thing you can't compromise on in F1 - track position. But the unusual demands of this track mean nothing can be taken for granted.