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.
