Quadruple world champion Sebastian Vettel's bid for a record eighth straight Formula One race win of the season picked up speed last night with the Red Bull driver fastest in free practice for the U.S. Grand Prix.

After sitting near the bottom of the timing sheets in a foggy and chaotic opening practice session, the 26-year-old German sizzled under the blazing afternoon Texas sun and lapped the Circuit of the Americas with a best time of one minute 37.305 seconds.

"The circuit was quite slippery today; I was happy with the car but you always know you can improve here and there," Vettel told reporters. "I think we got through the programme, we were able to test some things, some were good and some not so good, but we will see.

"Ferrari looked quick this morning and McLaren could be a surprise here and Lotus and Mercedes will be strong as normal."

Vettel, who won his fourth successive title in India last month and now has 11 wins this year, was 0.115 seconds faster than Australian team mate Mark Webber who cemented the Red Bull one-two.

No driver has ever won eight races in a row in a single season, but Vettel equalled Michael Schumacher's seven earlier this month in Abu Dhabi and is favourite to win again this weekend.

The two Red Bulls were followed by the two Mercedes drivers with Germany's Nico Rosberg third and Britain's Lewis Hamilton, winner of the inaugural race in Austin last year with McLaren, ready for another strong weekend in fourth.

"This track and city are just such a beautiful place to be racing," said Hamilton. "It was a pretty straightforward day although we still have some work to do to nail the set-up because it's not quite there yet.

"But all of us in the team are focused on ending the season on a high and taking that foundation with us into the winter."

QUICK KOVALAINEN

Heikki Kovalainen, the Finn who is replacing compatriot Kimi Raikkonen at Lotus for the last two races after the 2007 champion pulled out to have back surgery, was an impressive fifth fastest in the afternoon.

The Finn's French team mate Romain Grosjean was eighth in the same session.

Lotus are fighting with Ferrari and second-placed Mercedes for the coveted, and lucrative, runner-up slot in the constructors' championship already won by Red Bull.

"I didn't know what to expect to be honest, it was such a short notice so it is difficult to know but overall we had a pretty good day," said Kovalainen, who only got the call from Lotus on Tuesday evening.

"I felt comfortable straight-away in the car. I think we can still improve a little bit tomorrow to stay competitive."

McLaren had another tough day with Jenson Button handed a three place grid penalty for Sunday's race after stewards ruled the Briton had overtaken a car while the morning session was under a red flag.

He was ninth quickest in the afternoon ahead of Ferrari's Fernando Alonso, who was given the all-clear to race after a medical on Thursday.

The Spaniard, who hurt his back when he hit a kerb at high speed during the race in Abu Dhabi, had topped the delayed and truncated morning practice session but could not match that pace in the afternoon.

"I had no problem being back on track today. My back felt fine and I was able to drive the same as always," said Alonso. "Compared to last year, the track conditions seemed better."

Russian Daniil Kvyat made his first appearance for Toro Rosso at a race weekend and was 17th fastest with a lap only 0.202 slower than Australian race regular Daniel Ricciardo, the man he is replacing next season.

The day got off to a stuttering start when teams and drivers were kept waiting for 40 minutes as heavy fog shrouded the circuit carved out of the south Texas scrubland.

Formula One's governing body, the FIA, said the delay was necessary because medical helicopters would not be able to take off or land if needed.

When it did eventually start, the session was soon red flagged again with officials waiting for an absent helicopter to land.