Did Rosberg step up or Hamilton stumble?

Nico Rosberg badly needed this result. More than he let on, probably. And in a Formula 1 race where Mercedes comprehensively asserted itself over a resurgent Ferrari, it was imperative that Rosberg took a big step towards arresting the incessant momentum that team-mate Lewis Hamilton built up in the title race across the first four grands prix of the season.

So far this year Rosberg has looked a shadow of the driver who more often than not was faster than anyone else in the F1 field, and who led much of the 2014 world championship.

The advantage in that battle eventually slipped through his fingers of course. But Rosberg came into this campaign determined to be better, faster and stronger than he was last year. He used the winter to focus on improving his weak points - namely too often conceding races from winning positions, while maintaining his strengths - chiefly a knack of often being faster than Hamilton on Saturdays.

But it hadn't worked out that way thus far. Hamilton too spent his winter brooding on how to become an even bigger force than he already was. Sure, he was world champion for a second time, and yes he'd often been able to out-race Rosberg even when conceding the advantage of pole position, but Hamilton knew he needed to be better as well. After all, the holy grail of equalling his idol Ayrton Senna's tally of three world titles beckons...

Up until the Spanish Grand Prix Hamilton had been utterly relentless. More focused, faster, totally in control. He'd claimed pole for each of the first four races, and won three of them. But for Ferrari's opportunism in Malaysia, it would have stood as a Hamilton clean-sweep.

This time Rosberg put himself in a position to control the start © XPB

Meanwhile, Rosberg looked to be floundering in the face of his team-mate's gathering charge. He is not one to hide behind excuses, and freely admitted he simply hadn't been good enough in qualifying in Australia and Malaysia. In China he came agonisingly close to beating Hamilton, but in Bahrain he got his qualifying strategy wrong and again lost out.

By suddenly (and consistently) failing to deliver on Saturdays, Rosberg found himself badly compromised on Sundays. Ferrari's improved form also meant he (usually) could not simply focus on the car ahead, so missing out on pole effectively meant 'game over' in terms of his victory hopes.

After his most recent loss in Bahrain, where the compromise of being forced into a battle with Ferrari actually broke his brakes and meant defeat to Kimi Raikkonen as well, it looked as though Rosberg's season was fast unravelling already.

But in Spain Rosberg finally pulled everything together - beating Hamilton to pole (comfortably) and converting that into his ninth career victory. In fact, once Hamilton slipped behind Sebastian Vettel's Ferrari, after making a slow getaway from second on the grid, the race was effectively in Rosberg's pocket.

After the race, Mercedes non-executive chairman Niki Lauda reckoned Rosberg needed to get one over his team-mate to gain crucial ground in the mental battle that defines high-level sporting contests. "He did a perfect job," Lauda told AUTOSPORT. "He fought back and this is good for the team, and for Nico.

"If he didn't do it here then Monte Carlo would become a very different race and he would lose a little bit of momentum in his head. Thank God it's back."

Rosberg also conceded the significance of finally beating Hamilton this year - even if he didn't feel there was any particular reason he had an edge around the Circuit de Catalunya.

"It's difficult when Lewis wins in a consecutive manner like that, but of course I'm confident in my abilities and in my approach, so I did believe the win was going to come very soon," he said.

World champion Hamilton was the one with thinking to do in Spain © XPB

"It's just been a good weekend. Everything has come together and it hadn't come together until now. There was no different approach. Of course I'm always modifying, I'm trying to optimise my approach, make small changes here and there, but fundamentally everything was the same."

Except the final result. So if Rosberg did nothing special, why did Hamilton fail to reach Rosberg's level here?

The conditions were generally hot and windy and many drivers struggled to stop their cars sliding. Hamilton spoke after qualifying of his difficulty in dialling out oversteer from the W06 without shifting the balance too far the other way, while also suggesting Rosberg's own driving style was helping him avoid such problems.

Mercedes boss Toto Wolff reckoned the difference between the pair was "difficult to see".

"When you are on such a level playing field, it's about feeling comfortable in the car, comfortable on the track," he explained.

"Before the weekend Lewis mentioned Barcelona was not his favourite - I don't know if that was the reason, but it was very difficult for him to get the right set-up with the gusty winds and lack of grip from session to session.

Even with DRS, Hamilton couldn't pass Vettel on the track © LAT

"Obviously going into the race, the minute you are able to control it from the front it gives you a big advantage. This is what Nico did in a very controlled way."

That utter control meant the focus shifted to the battle for second. Mercedes pitted Hamilton before Vettel (on lap 13) in an effort to vault him back ahead of the Ferrari, but a cross-threaded wheelnut meant Hamilton lost almost three seconds to a slow left-rear change.

This delay gave Ferrari an opportunity to stop Vettel without losing track position to the Mercedes, so it pitted its man on the next lap to keep him ahead. Hamilton closed to within six tenths of Vettel by the end of lap 20, and used his DRS to get a run at the Ferrari heading toward Turn 1, but couldn't get close enough to pull a move.

"It's been a long time since I've had such a bad start," said Hamilton. "I was fortunate to keep third. I tried to fight, but this track isn't very good for overtaking - it's actually the worst for overtaking, it's impossible to follow here. For the guys at the front, it doesn't matter what you do, you can't get close enough."

At this stage Mercedes told Hamilton he would have to pass Vettel on-track if he wanted to get ahead, to which he replied: "That's pretty much impossible. Find another solution."

That solution was to switch Hamilton onto a three-stop strategy, which would allow the world champion to leapfrog Vettel later in the race - provided he could clear the slower cars he would inevitably fall behind after pitting out of sequence.

Mercedes pitted Hamilton for a second time on lap 32 and he came out just behind Raikkonen's Ferrari, which was running a long middle stint on hard tyres. With fresh hards fitted to his Mercedes, Hamilton breezed past Raikkonen using DRS at the start of lap 34 and retook third spot from the Williams of Valtteri Bottas on the inside line at Turn 1 on lap 39.

Vettel (on worn mediums) struggled to lap within a couple of seconds of Hamilton for much of that sequence, and Ferrari pulled its man in for his final stop on lap 40.

Rosberg made his own final stop five laps later, allowing Hamilton to lead the race. Many (including Rosberg!) wondered whether the #44 Mercedes might attempt a heroic 34-lap stint on its current set of tyres to try to snatch victory from the jaws of defeat...

But, as Rosberg put it, that "wasn't doable". Hamilton pitted for the final time on lap 51, allowing Rosberg to escape to his well-earned victory. Hamilton rejoined with several seconds in hand over Vettel's Ferrari, thus the Scuderia's hopes of denying Hamilton and Rosberg their 14th one-two result in F1 were finally dashed.

"I thought it would be tricky to keep him behind," conceded Vettel. "They were a fair amount quicker at the end of the race.

"Unfortunately, I came out in traffic [from the last stop]. I lost two or three seconds, which might have been just enough to stay in front. But with the speed they have it's fair to say they deserved to be in front."

Having finally cleared the Ferrari, Hamilton set about hunting down Rosberg. The #6 Mercedes led by 20.648s with 14 laps to run, meaning Hamilton needed to outpace his team-mate by roughly 1.5s per lap to still have a (small) chance of challenging for victory.

Hamilton had an optimistic go at catching Rosberg in the closing laps © XPB

Mercedes instructed Hamilton to hold position. "It's impossible to catch him?" he asked. The team told him Rosberg would simply up his own pace in response. Hamilton gave it a go anyway, but eventually realised the cause was lost and gave up the chase.

"Nico was quite far ahead by time I got onto the last stint - a huge amount of time to catch up," Hamilton said. "It was very hard to see if I could have the pace on him in order to close it. It was less than a second [difference in laptime] on occasions. I pushed until six laps to go and still had 13s [to make up], so I realised I should bring the car home and live to fight another day."

In recovering to second Hamilton ensured minimal damage to his championship lead (which Rosberg cut to 20 points with this win), while Vettel and the Ferrari camp were left to rue their heaviest defeat of the season so far.

Vettel's 45.342s deficit to the winning Mercedes was bigger even than in Melbourne, where the world decried the end of competition in Formula 1 as we know it.

Ferrari brought a significant upgrade to the SF15-T for this race, but Mercedes didn't leave the W06 unmolested either, and actually increased its pace advantage - to the tune of about three tenths in qualifying.

Perhaps Ferrari's massive upgrade package, which included basically "everything around the monocoque" according to one insider, wasn't delivering everything Maranello expected here?

"It's a good question," conceded Vettel. "If you look at the gap to Mercedes in the race, it's the biggest gap so far. It's not so good but I'm confident we are going in the right direction.

Vettel was back on the podium but a long way from victory © XPB

"For some reason, we are not so competitive here. Either this track doesn't suit us, or the conditions really suited Mercedes."

Ferrari boss Maurizio Arrivabene pointed out that his team was "losing half a second" in the final sector of the lap (Rosberg was 0.462s quicker than Vettel in qualifying in the section that incorporates Turns 10-16), suggesting that perhaps low-speed downforce and traction is what's mainly lacking from the SF15-T, which doesn't bode well for the next race in Monaco...

"Barcelona was not an easy track, it was not easy for the others too," argued Arrivabene. "The gap to Mercedes is still there. We need to analyse the gap to understand it.

"If something was wrong, we have to do the work and take it with humility. We need to understand, we need to find out how to improve the new package, but it's clear the new package is better than the old one.

"We have done a comparison and the solution is good, but in reality it's not good enough."

Rosberg pointed out after this race that Mercedes has actually scored more points after five races this season than it managed at the same stage last year (on account of not repeating Hamilton's rubber tube-induced retirement from the Australian GP of 2014).

That could become ominous if Mercedes can carry the supreme form it showed in Spain through the next sequence of races and leave Ferrari trailing.

Rosberg's aim will be to do something similar in the intra-team battle with Hamilton. Monte Carlo has been a happy (home) hunting ground for Rosberg in the past. He doesn't like talking about 'momentum' in a title race, but he could undoubtedly do with building some of his own.

His performance in Spain has certainly given him some very solid foundations on which to do just that.