Russian GP: Lewis Hamilton still in title race - Nico Rosberg

Nico Rosberg insists he still faces a tough fight from Mercedes team-mate Lewis Hamilton for the title despite winning the season's first four races.

Rosberg leads the British world champion by 43 points after Sunday's Russian Grand Prix.

"I am well aware Lewis is going to bounce back when he has a clean weekend and it will be the usual tough battle against him," the German said.

"Sport is all about ups and downs. I just need to mentally prepare."

Victory at Sochi was Rosberg's seventh in a row, dating back to last October's Mexican Grand Prix, the race after Hamilton had clinched his third world title.

Only three other drivers in F1 history have achieved that feat - Alberto Ascari, Michael Schumacher and Sebastian Vettel - and all went on to win the championship that year.

But Rosberg said: "The other people did not have Lewis Hamilton as their team-mate.

"Yes, I have a good points lead but there are 425 points to go and I have only a 43-point lead. It is completely not worth talking about.

"Lewis is driving as strong as ever. For sure he will bounce back massively. He doesn't have an issue with fighting back either with his head.

"I am sure we will see a lot more of the battle between us and it is always going to be a tough battle."

Rosberg joins an elite group of F1 drivers who have won seven races in a row

Rosberg admitted becoming only the fourth driver in history to win seven in a row "sounds cool".

However, he added: "It is not my focus. Yes, it is a great start but I just enjoy winning, race by race. Barcelona next."

Rosberg's advantage means Hamilton would have to win and finish second without the German scoring to draw level.

Hamilton finished second in Russia after starting 10th on the grid because of an engine failure in qualifying. The problem was in the same part that broke in China two weeks ago and left him at the back of the grid.

He said: "He has a buffer knowing even if he has two bad weekends he is still in the fight, which is a huge confidence boost, and generally he has been starting at the front with no-one to really bother him, so it has been a nice Sunday drive for him. But there are still 17 races to go and still 17 races I can give him hell."

But Hamilton said he was concerned about the likelihood of receiving grid penalties later in the season - because he was using up engines quickly and because he already has two reprimands and a third would mean an automatic 10-place grid penalty.

"It's not that it hurts," said Hamilton, who was closing on Rosberg in Russia before being told to back off because his engine was losing water pressure.

Nico Rosberg has been runner-up to Lewis Hamilton in the title race for the past two years

"I don't sit here all happy, because nothing has gone particularly well. I got myself back up there and wasn't able to fight for the win.

"I believe I had a chance to win. That is the only thing in my mind.

"Also I am running out of engines as the races go by. It is not hurting, it is just not great.

"I am aware there is likely to be one more 10-place penalty because I have one more reprimand to go. The last one was just ridiculous."

Mercedes F1 boss Toto Wolff said the team had "let Hamilton down with the engine" and that they would urgently try to get to the bottom of the problems.

"This is a mechanical sport and these things happen," Wolff said.

"We had a problem on the MGU-K drive on Nico's car and it looked as if he might not finish the race.

"We are pushing the limits a lot on the chassis and the engine a lot in order to have a competitive car and this is why we are winning races, but if you push the limits at a certain stage you go beyond them."

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