By Steve Keating
AUSTIN, Texas (Reuters) - Red Bull's Sebastian Vettel posted the fastest time in opening free practice for the U.S. Grand Prix as drivers got their first taste of Formula One's newest venue on Friday.
Vettel, who can clinch a third straight drivers' title on Sunday at the $400 million Circuit of the Americas, clocked a best lap of one minute 38.125 seconds around the 5.516km (3.4 mile) track carved out of the south Texas scrublands.
McLaren's Lewis Hamilton, who won the last Formula One race held in the United States five years ago at Indianapolis, was second fastest in 1:39.543 although times had little significance in a session that was more about familiarization.
Ferrari's Fernando Alonso, 10 points behind championship leader Vettel and the only driver who can deny his German rival the title, had the third best time in 1:40.337.
"The first reactions from the drivers are very positive," Red Bull's team principal Christian Horner told reporters. "It's been a positive start for us. We need to build on that momentum."
Vettel, 25, will be starting his 100th grand prix this weekend with Red Bull also poised to clinch the constructors' crown for the third year in a row.
The morning session, run in front of mostly empty grandstands, produced a few thrills as drivers came to grips on the slick undulating circuit.
While the 90-minute practice finished without major incident there were plenty of spins and more than a few forays into runoff areas as drivers tested the limits of the track.
"The circuit was very dirty this morning as you would expect with a new surface, so it took a while and you could see the evolution of the track was getting quicker and quicker every lap," said Horner. "And it's pretty cool here, I think probably cooler than all of us thought. So it's taken a few laps for the tires to cool up to temperature.
"But I think that as the rubber goes down and the drivers push harder, then that lap time will come much sooner."
Drivers will be back out on the track later on Friday for another 90 minute free practice where times will continue to drop.
Despite being a new circuit and the pressure of have a championship on the line, Horner said his team is trying to approach the U.S. Grand the same way it does every other race.
"The team's very focused," said Horner. "We're really concentrated on getting the most out of this weekend and trying not to allow other distractions to interfere with our preparation and performance.
"We've come here to do a job and then the championships will tend to take care of themselves.
"Our approach here is no different here to any other race. There's obviously a bit more hanging on it than others as we've got to the business end of the championship."
(Additional reporting by Corrie MacLaggan; Editing by Alan Baldwin and Gene Cherry)