World Champion Lewis Hamilton concedes that improvements need to be made to his Mercedes W07 Hybrid overnight after he commented that his car felt "tricky to drive" during practice for the Spanish Grand Prix.
Hamilton trailed Mercedes team-mate Nico Rosberg by over seven-tenths of a second during the afternoon practice session at the Circuit de Barcelona-Catalunya, with the pair split by Ferrari's Kimi Raikkonen.
"That wasn't an easy day for me, I struggled with the car balance through the day and this afternoon in particular it felt really tricky to drive," he said.
"I had traffic on my quick lap, so the lap times don't tell the full story, but it's clear we have plenty of work to do this evening to get the car driving like I want it to.
"But we will get our heads down this evening, work through the data and I'm confident we can pull things around for tomorrow."
Title leader Rosberg, who is bidding for his eighth successive win, was more encouraged after Friday's sessions.
"Times were looking good on the longer and also on the shorter stints," he said.
"I feel very comfortable with the car with some new parts added and they seemed to have worked well."