Barcelona F1 test: Nico Rosberg says more to come from Mercedes

The German, who finished runner-up in the title race last year, clocked a 1m22.792s on the soft tyres, which was 0.7s quicker than the previous best, set by Williams's Felipe Massa on the same tyre on Thursday.

But despite the impressive time, Rosberg said he still feels unsettled with the car's set-up.

"Today was a difficult day because even though the numbers looked like they were in the right place, actually the set-up was far away from where it needed to be," Rosberg told reporters after the day's running.

"I don't really understand that. The whole day we have been chasing the set-up and trying to find the right direction.

"It's really difficult to understand which way to go.

"I had a lot of oversteer in the car, so the rear was all over the place, and poor traction.

"The car was not good on the rear, the opposite of planted.

"I always want better and better so we are still a work in progress.

"Putting the soft tyres on, that gives the grip anyway so it doesn't really matter what the balance is.

"It would be more or less possible to do a good lap."

Rosberg, who has one more day in the car before the season-opener in Australia, added that the team made progress but it took longer than expected.

"The whole day we were going in the right direction but it took a long, long time because we were extremely far away," he said.

"That's very unusual and it caught us off guard a little bit.

"By the end of the day, we were in a decent place. I am pleased with my lap.

"It was a good ending. It's now just about fine-tuning."