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 with each setting the pace in practice sessions on Friday.

German Rosberg, chasing his 10th win of the season and 33 points clear of the 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, 0.315 slower.

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

"The running of the car was different between P1 and P2 (first and second practice) because of the temperature," said Rosberg. "It was very cold when we ran this morning.

"The weather also had a bearing on the balance of the car as it altered as a consequence of the changing wind direction.

"It should be different tomorrow because the weather is going to change again...and it means what we learn today might not necessarily help us for Saturday."

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.

GREAT DAY

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.

"It's been a great day for reliability and getting our program done," said Hamilton.

"Given the wet weather conditions here last year and the lack of reference points for each tire compound, Nico and I had to cover as much ground as possible to help with our understanding."

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 sat out the session before returning in the afternoon.

(Editing by Ed Osmond and Tony Jimenez)