Lewis Hamilton made a quick start to his US Grand Prix preparations with the fastest time in first practice for a key race in the title battle with Mercedes teammate and championship leader Nico Rosberg.
The triple world champion lapped the Circuit of the Americas, in gusty conditions, putting in a best time of one minute 37.428 seconds late in the session to knock the German off the top.
Rosberg, the title favourite who leads the Briton by 33 points with four races remaining, was second on the timesheets and 0.315 slower.
Hamilton has won three of the last four races in Austin, taking his third title at the Texan track last year, but he desperately needs another victory on Sunday to revive his flagging hopes of a fourth crown.
Even that will not be enough if Rosberg finishes the remaining races in the runner-up position, and the German has said he is not about to settle for second best as he chases a 10th win of the year.
Red Bull's Dutch teenager Max Verstappen was third fastest with Ferrari's Kimi Raikkonen fourth and Force India's Nico Hulkenberg fifth.
Verstappen's best time was still 1.6 seconds slower than Rosberg, albeit on medium tyres, with Mercedes - who clinched their third successive constructors' title in Japan two weeks ago - on softs and again in a battle of their own.
Ferrari's Sebastian Vettel, like Hamilton a past winner in Austin, was only eighth fastest. The German hit a kerb, smashing bodywork, and drove back to the pits one-handed while holding the remnants of his right side mirror in the other.
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.
