Why is Rosberg dominating?

Nico Rosberg's form in the final three races of the 2015 Formula 1 season was easily explained away. Lewis Hamilton had taken his foot off the gas after winning the world championship and Rosberg no longer had the pressure of a title fight on his shoulders.

Less easily accounted for is how the German has carried that form into this season. Hamilton started on pole in the first two races but Rosberg delivered when it counted on Sunday afternoon.

He now has five wins on the bounce and a healthy 17-point lead. We're only two races in and there are still 19 to go, but his performance is significant.

Is it down to Rosberg building on last year's momentum, getting in the right place mentally over the winter and then simply not making mistakes at the start of the campaign? Is it down to Hamilton's level dropping and him not getting back up to speed quickly enough? Or is it a combination of the two factors?

The 2015 season was a tough pill to swallow for Rosberg. Having run Hamilton close the previous year, he was blown away and was never a genuine title threat to his team-mate.

Rosberg admitted he had shifted focus to his race form, in a bid to try and get the upper hand on Sundays, but that had come at the cost of his qualifying performances.

He did get on pole for the last six races of 2015, but he buckled under the pressure of the title fight - typified when he was caught out by a "gust of wind" at Austin and gifted the lead and the title to Hamilton. When the pressure lifted, and Hamilton lifted off, Rosberg didn't put a foot wrong to win the last three races. Momentum is a powerful thing.

Speaking at Mercedes' pre-season press conference, Rosberg refused to be drawn on what those end-of-2015 race wins did for his confidence. But he was sharp and composed throughout the briefing while Hamilton seemed distracted, preferring to mess around on his phone and post videos on Snapchat.

Rosberg had his head in the game and needs to keep it there if he is to have any chance of beating Hamilton over the season. He fed off the momentum and has made a conscious effort to shift focus back to race day this season. He has accepted he's not as quick as Hamilton, but he is still quick enough to capitalise on any opportunity - especially when the pressure isn't on him at this stage of the season.

The German's contract is up at the end of the season and while he says he expects to be at Mercedes "for some time to come" and "it's not something that is on my mind at the moment", he knows he needs to deliver a stronger challenge to Hamilton this term to convince the team to extend his deal. There is also the threat from Pascal Wehrlein, who is impressing for Manor and waiting in the wings.

Mercedes chief Toto Wolff has said talks with Rosberg are unlikely to happen until the middle of this year as he first wanted to see how the season would pan out. It's a combination of good fortune and solid preparation, then, that Rosberg has been so strong out of the box both on and off the track.

Both Mercedes drivers have made poor starts to races in the opening part of the season, but it has been Rosberg who has recovered best while Hamilton has had to resort to damage limitation.

Hamilton says he doesn't care about his team-mate's winning streak. After all, Australia often produces a freak race while Rosberg always excels in Bahrain. But Rosberg has achieved the optimum results and the longer his run continues, the more likely it is to have an adverse effect on Hamilton.

The reigning champion remains title favourite and the only man who can beat him is himself. He was a bit shaky at times in 2014 as the title fight intensified and that is arguably his only weakness. But he managed to shake it off and come through, something Rosberg has yet to show the capacity to do.

Hamilton has been remarkably open this season and during testing admitted his focus eased once the title was won last year.

"My mind shifted after I won the championship last year," he said. "The intensity of the championship was over and I enjoyed it more than I have enjoyed the end of a season in my whole career both in the car and out.

"I still arrived and did the job in terms of scoring the points for the team and let's see where it goes from the beginning of this year. I feel very strong, I feel absolutely determined to get back in and win, that's just in my DNA.

"It's not like I have to go and re-invent myself and find extra motivation, it is just there thankfully, and I'm sure the competition is going to be stronger than ever this year and I have to make sure that I am stronger than ever."

That strength has not been immediately evident in the first two races, although his qualifying performances prove the speed remains. And his fightback through the field in both Australia and Bahrain proves Hamilton's determination.

But there have been too many struggles. He has been unable to find a balance he is happy with and he hasn't been able to get on top of the single clutch starts, which have twice ended his victory hopes before the first corner.

Whether or not Hamilton is being sincere when he says the bad start hasn't affected him is anyone's guess. But the longer Rosberg remains unbeaten, the more chance of it will have a negative impact on the reigning world champion. The extra work Hamilton might have to put in as a result could have an impact on his life outside of the track. That could also upset the equilibrium he achieved in his work/life balance last year.

Rosberg needed Hamilton's level to drop, and he's got that. But how long will it last?

Hamilton will get back on form and remains the favourite for the title. So Rosberg must capitalise and then do all he can to hang on if he is to have any hope of adding his name to F1's illustrious list of world champions.

The window in which he will have an opportunity to do so is rapidly closing. A third championship defeat to Hamilton may be the final nail in the coffin of his hopes.