Hamilton on brink of title but rivals are not giving up

Lewis Hamilton is now on the brink of wrapping up the 2015 Formula 1 world championships with four races still to run.

Meanwhile a late stewards' call after the Russian Grand Prix that demoted Ferrari's Kimi Raikkonen means that Mercedes has secured the constructors' crown for the second consecutive time.

Sochi winner Hamilton's teammate Nico Rosberg started Sunday's race from pole, but he ultimately retired with a broken throttle.

It means that if the Briton scores just 2 points more than Rosberg at Austin in two weeks, the German will be out of the running for the drivers' title.

"It's pretty incredible sometimes how tough F1 is," said Rosberg.

Asked if the title is now gone, he answered: "I don't know. But I am never going to win a championship when normal things like that break."

Hamilton, however, refused to "get ahead of myself" by counting on his third drivers' championship already, but Sebastian Vettel acknowledged that Ferrari's real challenge will probably have to wait until 2016.

"Of course you never give up," said Vettel, "otherwise what is the point of even being here?"

But he is more hopeful that another step forward for the Maranello team could result in a proper title challenge in 2016.

"I hope so," said the German. "That is our target. We want to be a bit better and make sure we give Mercedes a really good challenge."

Hamilton leads the points standings with 302 points to his credit and 66 more than Vettel in second, while Rosberg has dropped to third in the standings with 229 points.