AUSTIN (AFP) - Championship leader Lewis Hamilton topped the times ahead of his Mercedes team-mate and title rival Nico Rosberg in Friday's opening free practice session for Sunday's United States Grand Prix.

The 29-year-old Briton clocked a best lap time of one minute and 39.941 seconds to wind up two-tenths of a second clear of the German after the pair took their time to exert themselves under the vast blue skies at the Circuit of the Americas.

Hamilton leads Rosberg by 17 points with three races remaining including the final Abu Dhabi Grand Prix where double points will be awarded.

Jenson Button, the 2009 champion, was third fastest in the leading McLaren ahead of Russian Daniil Kvyat of Toro Rosso, Dane Kevin Magnussen in the second McLaren and two time champion Fernando Alonso of Ferrari.

Last year's winner and four-time champion Sebastian Vettel was seventh ahead of Felipe Nasr, making an impression as the Williams test driver, Nico Hulkenberg of Force India and Dutch teenager Max Verstappen, back in action for Toro Rosso, after his first outing in Japan.

Vettel's Red Bull team-mate Daniel Ricciardo suffered an engine failure during the session and wound up only 17th of the 18 cars on the smallest F1 grid for a decade since the Monaco Grand Prix.

Both the Marussia and Caterham teams were absent after withdrawing due to financial and other problems.

Vettel may miss much of Saturday's qualifying session because he also has been forced to change engines while Button, and Vettel, have been given five-place grid penalties for gearbox changes.

Vettel is expected to start Sunday's 56-lap race from the pit lane.