Lewis Hamilton said he was unsure why he was off the pace in the second free practice session of the weekend in Korea.
The McLaren driver led the way in FP1 in Yeongam but failed to replicate the performance in the second 90 minute session of the weekend.
Instead, Hamilton was unable to match his earlier pace and came in eighth quickest, eight-tenths of a second off the pace, and he admitted he was unsure why he had gone backwards on the times.
“I struggled a little bit with set-up today,” he said. “There was quite a big difference in feeling between FP1 and FP2, which we don't fully understand yet. This morning's session was fine, but I made some changes for the afternoon and wasn't quite able to get a good time out of either the Prime or the Option.
“So now we'll work hard to figure out what direction we want to take. The Red Bulls look pretty quick, but Jenson wasn't that far off their pace this afternoon. So that's good. We'll make some changes to the balance overnight, and hopefully we'll be in better shape for tomorrow.
“As I say, this afternoon Jenson showed that our car has pace; all we need to do is fully extract it. If we can improve the balance overnight, I think we can be competitive.”
Team boss Martin Whitmarsh admitted it was difficult to nail set-up at the Korean circuit but said he felt the team was moving in the right direction.
“Finding the perfect set-up for this circuit is a somewhat tricky balancing act – and it's not made any easier by the 'green' track surface and the changing wind direction between sessions,” he said. “Nevertheless, I think we've made a solid start to the weekend. We evaluated a number of new solutions and made decent progress towards cracking the balance needed for both the Soft and the Supersoft tyres.
“As always, the sharp end of the field looks extremely competitive, so the remainder of the weekend will be all about trying to maximise the slightest competitive advantage. There's still work to be done, but our engineers and mechanics will doubtless rise to the challenge.
“We're in the thick of it, and we're very much looking forward to qualifying tomorrow.”