By Alan Baldwin
AUSTIN, Texas (Reuters) - Formula One title favorite Lewis Hamilton shattered the Circuit of the Americas track record on Friday as he dominated practice for a U.S. Grand Prix that could bring him a fourth world championship.
The Mercedes driver had been half a second quicker already than his Ferrari rival Sebastian Vettel in a damp opening session and dazzled in the afternoon as the sun made an appearance.
His storming time of one minute 34.668 seconds was the fastest ever at the Texas track and the Briton, winner of four out of five races to date there, was the only driver on the timesheets below the 1:35 mark.
Hamilton, 59 points clear of Vettel with four races and a total of 100 points remaining to be won, took pole in Austin last year with a record lap of 1:34.999.
Red Bull's Max Verstappen, fresh from signing a new contract through to the end of 2020, was second fastest in 1:35.065 with Vettel third in 1:35.192.
"It's been an interesting day with lots of ups and downs in terms of how the track has moved around and how the car feels. But overall the car was feeling good," said Hamilton, who could count on plenty of local support.
"There are lots of areas we can improve on, but generally it feels like a solid platform and a solid start to the weekend."
Hamilton's morning time was 1:36.335 seconds on the supersoft tires.
Vettel had a difficult afternoon, spinning into the gravel and complaining that the front of the car felt "like jelly", a problem that cost him precious track time while mechanics worked on the car.
"It was a messy afternoon and not an easy session but the car is quick so no need to worry," the German told reporters.
"Obviously we lost a lot of time but you need to take precautions. Now we need to have a good look but I'm sure for tomorrow we'll have more planned running and less trouble," he added.
Hamilton, who has won all but one of the five races since the August break, is clear favorite to triumph again on Sunday but he will need to score 16 points more than Vettel to clinch the title.
That means finishing first or second on Sunday, and hoping Vettel has a nightmare for the fourth race in a row. Otherwise the championship fight will remain open to the next round in Mexico.
While much of the attention was on the title rivals, Spaniard Carlos Sainz and New Zealand debutant Brendon Hartley also caught the eye with strong performances.
Sainz, moving to Renault from Toro Rosso in place of the dropped Briton Jolyon Palmer, was quicker than German team mate Nico Hulkenberg in both sessions.
Hartley, who will have a 25-place grid penalty for his debut, stepped into a single seater for the first time since 2012 and looked at home straight away with the 14th best time of the morning.
He was 17th in the afternoon, with Russian team mate Daniil Kvyat 1.2 seconds quicker in 13th.
The New Zealander, a Le Mans 24 Hours winner, did more laps (13 more than the 56 lap race distance) than anybody else as he drove an F1 car for the first time since 2010 when V8 engines were still around.
McLaren's Fernando Alonso made up lost time after a hydraulics problem in the morning with the seventh best time.
(Reporting by Alan Baldwin, editing by Rex Gowar)