Lewis Hamilton is wary of the speed of the Red Bull Racing team after its dominant display in second practice in Spain.
Sebastian Vettel led teammate Mark Webber in the afternoon session, as the Red Bull duo finished nearly eight tenths of a second ahead of their closest rival. Although Hamilton topped the times in the morning, he reckons the Red Bull duo are still far ahead.
"It's not been bad," said Hamilton, fifth quickest. "It's definitely a step forward but it could be interesting tomorrow to see how big a step exactly we've made compared to others.
"It seems that maybe one team seems to be very, very quick at the moment. We just have to keep pushing and trying as hard as we can to close that gap."
Teammate Jenson Button was not completely satisfied with his day's work after finishing down in eighth position.
"I think we've still got a little bit of work to do, to be honest," he said. "I didn't get as much running done this morning as I would have liked because I hit something coming out of the pits – probably a small part of a car, which damaged one of my car's bargeboards. This afternoon we embarked on some setup changes that we thought were headed in the right direction, but I'm not sure if they were.
"So there's a lot of work still to be done on both tires – some of it is possibly setup, and some of it is getting the tires in their correct working range."