Rosberg on top after first day of U.S. practice

By Alan Baldwin

AUSTIN, Texas ( Reuters) - Formula One championship leader Nico Rosberg and Mercedes team mate and title rival Lewis Hamilton limbered up for their U.S. Grand Prix duel on Friday with each setting the pace in practice sessions.

German Rosberg, chasing his 10th win of the season and 33 points clear of the reigning triple world champion, ended the day with the fastest time after lapping the Circuit of the Americas in one minute 37.358 seconds.

Hamilton, winner in three of the last four races at the Texan track including 2015 when he took his third title, was quickest in the morning in 1:37.428 with Rosberg second and 0.315 slower.

Briton Hamilton dropped to third in the afternoon, 0.291 behind Rosberg, with Red Bull's Australian Daniel Ricciardo splitting the dominant Mercedes pair.

Rosberg and Hamilton are the only drivers still in contention for the title with four races remaining while Mercedes have already clinched the constructors' crown for the third year in a row.

Hamilton needs another victory on Sunday but knows that even winning all the remaining races will not be enough if Rosberg finishes them in the runner-up position, and the German has said he is not about to settle for second best.

Red Bull's Dutch teenager Max Verstappen was third fastest in the morning with Ferrari's Kimi Raikkonen fourth and Force India's Nico Hulkenberg fifth.

Ferrari's Sebastian Vettel, like Hamilton a past winner in Austin, was eighth in the opening session after hitting a kerb, smashing bodywork, and driving back to the pits one-handed while holding the remnants of his right side mirror in the other.

The German came back strongly after lunch and was fourth, with Verstappen fifth.

Britain's Jordan King made his Formula One practice debut with Manor Racing, setting the 20th fastest time out of 22, while Germany's Pascal Wehrlein sitting out the session before returning in the afternoon.

Three drivers - Hulkenberg, Manor's Esteban Ocon and Williams' Valtteri Bottas - tried out the new 'halo' head protection device on their cars early in the session.

(Editing by Ed Osmond)