Nico Rosberg believes Mercedes' rivals have “stepped up their game” in 2015 with Ferrari having "surprised" the world champions.

Although the long-held expectation remains that Mercedes will again be the team to beat in the new season, it has been their rivals who have grabbed the headlines by setting the pace on the testing timesheets in Spain so far.

Ferrari, winless for the first time in two decades in 2014, have caught the eye with their SF15-T having finished in the top two on all six of the hithero testing days.

While they comfortably lead the mileage charts, Mercedes, in contrast, have yet to head the order – although arguably the first true glimpses of the W06’s expected strong pace was seen on Day Two at Barcelona as, on the slower medium tyres, Lewis Hamilton lapped within 0.4s of the session’s soft-tyre benchmark.

Nonetheless, after enjoying a near monopoly on the order in the first year of F1’s new turbo era, Rosberg is taking the apparent revival of the chasing pack seriously.

“It’s clear that the others have really stepped up their game,” the German told reporters.

Lewis Hamilton completed 89 laps on the second day of testing in Barcelona Lewis Hamilton completed 89 laps on the second day of testing in Barcelona

“The lap times are surprising to us and in relation it seems that Ferrari have made the biggest step.”

Asked if the Scuderia could even challenge them for wins at the start of the racing season, Rosberg replied: “Our eyes are wide open.”

The smart money remains on Mercedes picking up from where they left off in 2014 and leading the field at next month’s Australian GP. Certainly, the lap times of Hamilton and Rosberg – who shared driving duties on Day Two – on the medium rubber appeared somewhat ominous. While the world champion’s quicker 1:24.923 lap came on a short run, Rosberg’s 1:25.556 was set at the start of 12-lap stint when his W06 would have been heavier with fuel.

Lewis Hamilton returns to the Barcelona paddock after suffering a fever on the first day of the test

Mercedes split their Friday running as a precaution after Rosberg tweaked a nerve in his neck ahead of this week’s test. The 29-year-old explained that a change to his driving position in the new Mercedes cockpit had unexpectedly caused the problem.

“I’m just being careful. I know the cause of it – it was my position in the car, which I got a bit wrong. I didn’t realise. I changed it in the winter," Rosberg said.

"It’s incredible how fine a line it is; it’s centimetres in this case. I have to get used to it, it’s a bit different. I went even further into a safe direction, so it’s quite a big change.