Austin FP2: Rosberg on top chased by Ricciardo

Formula 1 championship leader Nico Rosberg and Mercedes team mate and title rival Lewis Hamilton limbered up for their United States Grand Prix duel on Friday with each setting the pace in the day's two practice sessions.

Rosberg, chasing his 10th win of the season and 33 points clear of the reigning triple F1 world champion, ended the day with the fastest time after lapping the Circuit of the Americas in 1:37.358 seconds.

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

But in the afternoon session Hamilton dropped to third on the timing sheets, 0.291 of a second behind Rosberg, with Red Bull's Daniel Ricciardo splitting the dominant Mercedes pair.

Rosberg and Hamilton are the only drivers still in contention for the 2016 world 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 fastest, with Verstappen fifth.

McLaren had a good afternoon, as Jenson Button just pipped team mate Fernando Alonso, 1:38.713 to 1:38.801, as Raikkonen completed the top 10 with after struggling with setup on his Ferrari.

Britain's Jordan King made his Formula 1 free 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.

Notably, during the long run simulations, both Red Bulls both looked very strong on the Pirelli soft tyres, while Hamilton got good performance on the mediums.

FP2 was briefly red-flagged to allow marshals to clear carbon fibre in Turn 4.