Rosberg expects big step from Mercedes

The German ended the first day of testing at Barcelona with the third-fastest time behind Mark Webber's Red Bull and Nico Hulkenberg's Williams, and he said that despite Webber's advantage of nearly one second, he feels the front of the pack is closely matched.

"It's difficult to say exactly where we are," Rosberg told reporters once testing had finished. "I think it's very close at the front, but that could all change again for Bahrain because the update we have is such a big step."

The former Williams driver added that the team's preparation for this season was being affected by the fact that its updates for the first race will be so wide-ranging.

"We did a lot today, made a lot of set-up changes, and we learned how they affected the car and how the car behaves on a long run," he said.

"The only thing is that you can't really work on details in terms of set-up because the car is going to change a lot for Bahrain, so there is no point in working on the small details now to maximise the performance."