Nico Rosberg reckons Mercedes' rivals have "really upped their game" over the winter and he believes Ferrari has made the "biggest step".

Ferrari has shown impressive pace so far in F1 2015 pre-season testing, with Kimi Raikkonen and Sebastian Vettel always at, or near, the head of the timesheets.

Mercedes in contrast, have been going for laps, with Rosberg and team-mate Lewis Hamilton combining to do 155 on Friday, day two of the Barcelona test.

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"It is clear in the way that the others have really stepped up their game - the lap times are surprising to us," Rosberg told reporters, including Crash.net.

"And in relation to us it seems that Ferrari has made the biggest step.

"Definitely our eyes are wide open at the moment."

Rosberg opted to sit out the afternoon session on Friday due to a trapped nerve in his neck, but added that Mercedes is making progress and things are "going to plan".

"It is a different circuit with this new car so it felt a bit different, but in general it was okay [this morning]," he continued. "I wasn't so happy with the car [at first] but then we found our way quickly with the setup and in the end it felt pretty good.

"At the moment we are just focusing on getting the miles done and working on setup. Of course, I want to go out and do qualifying laps all the time - but there is a right time for everything. We don't know where we are yet but things are going to plan.

"The development curve is still very steep. There is so much still to discover with these regulations in both the car and the Power Unit, so it is a big challenge. Overall, though, it's been a good start."

So is it happy?

"Never happy, no - but I am confident we will get things done the way we want. Things are improving, yes," Rosberg concluded.