Sergio Perez says he struggled with brake issues during second practice for the Chinese Grand Prix on Friday.
The Mexican, who starred in the previous race in Malaysia where he finished second, was fourth fastest in the morning session but dropped down to 11th in the afternoon.
Perez admitted he was happier with his car's balance in the morning, and blamed brake problems for a big lock-up that damaged his tires in the afternoon.
"The balance of my car had actually been better in the first free practice session compared to the afternoon," said Perez. "I wasn't too happy in the second free practice. I had problems with the brakes, which didn't work the way they should, and I lost too much front tire temperature on the back straight.
"It was too easy to lock the front. We need to improve the car's overall balance for qualifying. We have a bit of homework now, but we have gathered quite a good amount of data."
Teammate Kamui Kobayashi, on the other hand, was happier with his car's handling in the afternoon after finishing seventh fastest.
"We had a few minor issues to fix, but overall today's practice sessions went smoothly," he said. "Sometimes the visibility was a bit poor and this didn't help to predict where the track was and wasn't slippery.
"My car definitely felt better balanced in the afternoon's practice session than it did in the morning, and I believe we look okay here on this track.
"Of course there is still room for improvement in regard to the set-up. When we make our decisions for tomorrow we will not focus too much on today's track conditions, as we expect the temperatures to be a bit higher then."