Lewis Hamilton says the balance of the car was holding him back during practice for the Bahrain Grand Prix.
Hamilton was only tenth quickest in second practice.
"We worked as hard as we could today but couldn't quite get the balance of the car to where it needs to be," he said.
"I had some issues in P1 which we were able to improve for this afternoon so that's a positive, but we need to identify where we are losing time and find more performance."
"We have changed the set-up of the car to what we thought would suit this track but we need to look at that and make sure it is the right direction to go. The heat here is so hard on the tyres so another positive is that we did as many laps as possible to look at long runs and race preparation. We're not as far off as we might look today but there is a lot of hard work ahead and it will be a tough battle for us this weekend."
Team-mate Nico Rosberg admits that Mercedes is in for a tough weekend.
"It's clear that we weren't as strong as on the Fridays of the previous three race weekends," said Rosberg.
"The car had a lot of understeer and the race will be very tough on the rear tyres. The problem is that when you want to save the rear tyres, you try to set up the car with more understeer and that costs you time over one lap. So it's all about finding the right compromise."