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.

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.

(Editing by Alan Baldwin)