Lewis Hamilton was left confused by his McLaren's poor performance in free practice two in Korea.
The former world champion was eighth fastest, half a second down on his team-mate Jenson Button in fourth place.
"The first session seemed to be fine, then in the second one we made some set-up changes and I wasn't able to get the time out of the option tyre," said Hamilton.
"The first session was fantastic. I don't really know what's happened in this session. The wind direction's changed but it didn't seem to affect anyway else.
"We've got time, we just have to be calm tonight and try to figure out where we want to go with the settings of the car.
"The car definitely has some pace, we just need to extract it."
Hamilton said understeer was his main concern.
"There's a lot of understeer, particularly at the end of the last section," he said. "That's where I was losing most of the time."
Team-mate Button was generally optimistic about his car.
"It's always so tricky to get a good set-up around here, but we've made progress from this morning, when I wasn't entirely happy," he said.
"But this morning was also testing new parts and making sure everything was working correctly - it wasn't about set-up work."