Nico Rosberg says that he has not countenanced the possibility of finishing third in this year's F1 world championship, despite having slipped behind Ferrari's Sebastian Vettel after failing to finish in Russia.

Having come within one race of the title last season, Rosberg has had a harder time of it in 2015, with Hamilton appearing to have raised his game, especially in qualifying, and having the rub of the green with Lady Luck. As a result, the Briton could take title number three at this weekend's USGP in Austin - he needs to score three points more than Rosberg and nine more than Vettel - with fully three rounds remaining on the slate.

While Rosberg is mindful that his quest could be over come Sunday, he typically refuses to concede - to the media at least - and insists that the possibility of not even completing a Mercedes 1-2 has yet to enter his mind.

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"I have nothing to lose," he maintained, "I want to win - and I have the opportunity to win - so that's a great target to have this weekend. I'm focused. That's it.... That's a good state of mind, no?

"I've never thought about finishing third because, with my car, I'm still on for first at the moment and that's what I'm thinking about. Even if I know that it's tough because I am also realistic and know that I need a lot of luck, I am fixed on that until you tell me that it's impossible."

Ferrari's engine upgrade, introduced after the summer break, has brought the Scuderia into sharper relief in Mercedes' mirrors, with Vettel adding a comfortable win on the streets of Singapore to a string of podium finishes in recent weeks. The four-time world champion currently lies second to Hamilton, seven points ahead of Rosberg, with four races to run, but will take an engine penalty this weekend in order to run the very latest version of Ferrari's V6.

"It makes it more simple for us across the weekend, but they're still a threat even from where they are [starting] because you never know with the weather or whatever," Rosberg insisted when asked whether Vettel's decision helped Mercedes take a decisive step towards fulfilling its championship ambitions.