- Sebastian Vettel dominates practice at United States Grand Prix
- F1 championship leader fastest in both morning and afternoon sessions
- Red Bull star can clinch third straight world title this weekend
- Nearest challenger Fernando Alonso third fastest for Ferrari in second runs
Sebastian Vettel made the perfect start in his bid for an historic world title triple by setting the fastest time in both practice sessions Friday for the United States Grand Prix.
Formula One is returning to the U.S. for the first time in five years on the new Circuit of Americas in Austin, Texas and the Red Bull star wasted little time in setting the pace.
The 25-year-old German can become the youngest three-time champion and only the third to achieve the feat after Juan Manuel Fangio and Michael Schumacher this weekend.
He leads Ferrari's Fernando Alonso by 10 points with two races remaining and if he gains 15 points more than the Spaniard on Sunday the title will be his.
Vettel had problems with a water leak at the start of the second session and had to take to the escape road on Turn 12 on one of his runs, but his best time of one minute 37.718 seconds took him clear of the pack.
Teammate Mark Webber of Australia was second quickest with Alonso third ahead of the McLaren pair of Lewis Hamilton and Jenson Button.
Kimi Raikkonen, who won the last round of the F1 season in Abu Dhabi, was only 11th fastest in his Lotus.
Earlier, Vettel was quickest in the first competitive F1 action on the 5.516km track with a time of one minute 38.125 seconds.
He was nearly one and a half seconds faster than Hamilton, but all the drivers were cautious as they adjusted to a slippery and unknown track, which had not been "rubbered" in.
It was the biggest margin at the end of any practice session this season to underline his recent dominance.
Saturday sees qualifying to decide grid positions for the race proper.