World Championship leader Lewis Hamilton says he needs to find "a lot" of performance on Saturday if he is to fight for victory in the British Grand Prix at Silverstone.
The Briton was unable to match the pace of teammate Nico Rosberg in practice on Friday, finishing down in fourth as the German led both sessions despite car problems in the morning.
Hamilton was nearly half a second off Rosberg's pace in FP2.
"Hopefully a lot," he said when asked how much more was to come, "because if there is not then we are in trouble.
"It hasn't been the easiest of days, but it has generally been okay. The long-run pace was not so great. We have a bit of work to do on the set-up.
"I have to sit down and work out where it is: it is a general balance thing that we have to get right."
Despite his pace-setting performances, Rosberg admitted he was not too pleased with the handling of his car either.
"No, not that happy," he said. "The car was all over the place still. I think it was the track out there that was really difficult, especially as it was a lot hotter now than it will be on Sunday for the race.
"So we need to be careful with that. The balance that was good today will not be good for Sunday by far, so that is where the difficulties come in and you have to really think ahead.
"But I think it was valuable today and learned a lot of things."