Sebastian Vettel plays down first place in practice

Red Bull's Sebastian Vettel has played down the significance of his finishing first in the second practice session for the United States Grand Prix.

The 25-year-old was 0.757 seconds clear of the field, with his teammate Mark Webber and Ferrari's Fernando Alonso finishing second and third respectively.

"[On] Friday you need to be careful [of the times]," BBC Sport quotes the German as saying.

"The fact that it was very slippery can make a bigger difference than you think. There were one or two surprises in terms of pace.

"It is important we are amongst the guys at the top and we [will] try to improve the car for tomorrow to make sure we stay there."

Vettel leads Alonso by 10 points in the race for the world championship title.