Lewis Hamilton says he is not enjoying driving his Mercedes after finishing fifth fastest in second practice for the British Grand Prix.
The Briton, 28, was seven-tenths of a second behind team-mate Nico Rosberg, who went quickest at Silverstone.
"I'm not happy with the balance of the car," said the 2008 world champion.
"I can't remember the last time I've felt so uncomfortable. I don't really know what it is but we'll work on it and we'll fix it."
Hamilton ended a 14-year association with McLaren when he joined Mercedes at the start of the season, and has since secured three podium finishes in seven races to sit fourth in the drivers' standings.
However, he has struggled with the feel of the car and has failed to mount a serious challenge for victory, something his team-mate Rosberg achieved in Monaco.
"I'm not enjoying the car at the moment," said Hamilton.
He appeared downbeat on Friday after a day which saw first practice effectively washed out and a limited amount of dry running in second practice.
"When you can't hook up Becketts and Maggots (corners), the rest of the track doesn't feel so good," he added.
"I've got to go back and work on the set-up. It's just some bottoming with the car which is making it feel a bit awkward.
"But I'll do my best. Saturday is going to be a better day."
Hamilton said his long run "didn't feel too good", but when comparing the race-simulation averages from second practice, the Briton matched rivals Fernando Alonso, of Ferrari, and both Red Bull drivers, Mark Webber and Sebastian Vettel.
Rosberg, who has been on pole three times this season, was optimistic about the car's qualifying speed but less positive about the race pace - something the team continue to struggle with.
"Over one lap, the car is still quick so qualifying should be OK," the 28-year-old German said. "We practised race speed today and it seemed decent but for sure it's going to be a little bit of a problem for us.
"It's going to be 10 degrees warmer on Saturday and Sunday so that could change the picture again and probably making it more difficult for us."