US GP: Vettel feels prepared despite lack of running in FP2

The world champion was called into the pits just three laps into the afternoon session and could not rejoin until the final half-hour.

While most of the field's lap counts were in the mid-30s region, Vettel managed just 16 tours - although he still set the pace by a commanding 0.757 seconds.

Vettel did not think the problem will have any impact on the rest of his United States Grand Prix weekend.

"We got into the rhythm really quickly," he said.

"This afternoon we had a bit of a problem and did not run as much as we would have liked to, but nevertheless we had two good runs - so we are well prepared for tomorrow."

Although Vettel was fastest in both Friday sessions and was backed up by team-mate Mark Webber in a Red Bull one-two in second practice, the German said the day one times could turn out to be totally irrelevant.

"On Friday you need to be careful and the fact it is so slippery can make a big difference," he said.

"I think there were one or two surprises today - and we will see what happens tomorrow. The track will constantly improve.

"It is important that we are among the guys at the top, and we'll try and improve the car and go from there."