Lewis Hamilton continued his imperious form by posting a record-breaking lap of the Circuit of the Americas en route to completing a practice double for the United States Grand Prix.
Hamilton, who will secure his fourth world championship here this weekend if he wins and Sebastian Vettel fails to finish inside the top five, dominated both sessions on Friday.
His best time of one minute and 34.668 seconds saw him finish comfortably clear of Red Bull's Max Verstappen, who earlier announced he was staying at Red Bull for a further three years, while Vettel was third in the order, half a second down on his championship rival.
Vettel looked on course to take Hamilton down to the wire this year following a string of impressive performances in the first half of the season, but his challenge has imploded since the summer break and he has failed to finish two of the last three grands prix.
The Ferrari driver, now 59 points behind Hamilton with only four rounds remaining, crashed out in Singapore before technical gremlins forced him to retire last time in Japan.
And while the four-time champion avoided any mechanical glitches here on Friday, he completed only 11 laps - 15 fewer than Hamilton - in the day's second session following a high-speed spin.
Vettel lost control of his Ferrari at the penultimate corner before ending up in the gravel. The German avoided any contact with the barriers, but following the detour, he complained his car felt like jelly.
His best lap was well adrift of Hamilton's super-quick effort, and his lack of track time will be of concern to both Vettel and his Ferrari team.
For Hamilton, there were no such dramas on a near-perfect day as he bids to become only the fifth driver in the history of Formula One to win more than three championships.
Hamilton's Mercedes team-mate Valtteri Bottas finished fourth in the order, 0.6 seconds adrift of the Briton, with Red Bull's Daniel Ricciardo fifth and Ferrari driver Kimi Raikkonen sixth.
Fernando Alonso, who will remain at McLaren next season, failed to complete one lap in the opening session following a hydraulics issue, but the two-time world champion finished seventh later in the day.
Brendon Hartley, the New Zealander making his grand prix debut here this weekend, finished 17th for Toro Rosso, more than three seconds off the pace.