Vettel heads Webber as Red Bull dominance continues

Vettel, who started from pole at the inaugural race in Austin last year, will be favoured to do so again on Sunday after lapping the Circuit of the Americas in a best time of one minute 36.733 seconds.

The 26-year-old German, who clinched his fourth successive title in India last month, will be chasing a Formula One record eighth win in a row on Sunday at the spectacular circuit carved out of the south Texas scrubland.

Vettel's Australian team mate Mark Webber was the only other driver to dip under 1:37.00, crossing 0.203 behind in a practice session run in overcast conditions.

Britain's Lewis Hamilton, a winner in Texas last year for McLaren, signalled he is ready to defend his U.S. title with Mercedes after posting the third best effort ahead of Germany's Nico Hulkenberg in a Sauber.

Frenchman Romain Grosjean, yet to win a race, was fifth fastest for Lotus followed by McLaren's Jenson Button, who will go into qualifying later on Saturday with a three spot grid penalty for passing under a red flag during Friday's opening practice session.

Ferrari continued to struggle to find pace on the twisting circuit, with Spain's former-world champion Fernando Alonso well down the timing chart in 11th with his Brazilian team mate Felipe Massa in 17th.