Sebastian Vettel maintained his total domination of practice at the United States Grand Prix on Saturday morning when he topped the times for Red Bull for a third successive session ahead of Sunday's showdown, his 100th race in Formula One.
The 25-year-old German, who needs to outscore nearest rival Fernando Alonso of Ferrari by 15 points to become the youngest triple champion in F1 history, clocked a fastest lap in one minute and 36.490 seconds to emerge fastest.
That was enough to leave him nearly three-tenths of a second clear of nearest challenger Lewis Hamilton who, in his penultimate race weekend with McLaren before joining Mercedes next year, recorded a best time of 1:36.748.
Both men clocked their times in a busy final flurry of activity in the final minutes of the hour-long session run in cold conditions, but in front of a big crowd dressed in winter jackets, at the new Circuit of the Americas 25 kilometres out of downtown Austin.
Venezuelan Pastor Maldonado was third fastest for Williams ahead of Alonso, Nico Rosberg of Mercedes and Felipe Massa in the second Ferrari.
Australian Mark Webber was seventh in the second Red Bull ahead of Sergio Perez of Sauber, who crashed into Frenchman Charles Pic's Marussia during the session, Nico Hulkenberg of Force India and Jenson Button in the second McLaren.
Bruno Senna was 11th for Williams ahead of retirement-bound seven-time champion Michael Schumacher of Mercedes, and Abu Dhabi Grand Prix winner Kimi Raikkonen of Lotus, who was 13th.
The Finn's Lotus team-mate Romain Grosjean was unable to run in the final part of the session on light fuel and softer tyres due toa gearbox failure.