Mercedes duo Lewis Hamilton and Nico Rosberg both shrugged off health issues on a Formula One testing day when Red Bull finally showed some speed.

Reigning champion Hamilton completed just 11 laps on the first day of the second pre-season test in Barcelona on Thursday before stepping out of his Mercedes complaining of "a high fever".

Ordinarily team-mate Rosberg would have taken over the wheel, but he was unable to deputise as he was struggling with an inflamed nerve in his neck.

But Rosberg, who on Friday tweeted a picture of himself and wife Vivian confirming she is expecting a baby girl in August, was back on track in the morning.

Come the afternoon Hamilton had also declared himself fit for duty, with the duo racking up a combined total of 155 laps - the Briton 89 and Rosberg 66 - the highest of any of the nine teams on show.

But as has become the norm so far in pre-season, current constructors' champions Mercedes are playing their pace card close to their chest.

Hamilton was only fifth quickest, 0.349secs behind Daniel Ricciardo in his Red Bull, with the Australian's best of his staggering individual tally of 142 laps being a one minute 24.574secs.

Rosberg, meanwhile, was almost a further six tenths of a second off the pace, but that is not a concern as Mercedes have been focused on reliability rather than outright speed.

Behind Ricciardo was Kimi Raikkonen in his Ferrari by one hundredth of a second, keeping up the Maranello marque's record of being in the top two on the timesheet over the six test days to date.

As far as Rosberg is concerned, Ferrari appear a clear threat.

"Definitely our eyes are wide open at the moment as it's clear the others have really stepped up their game," he said.

"Lap times are surprising to us. In relation, it seems Ferrari has made the biggest step."

Williams' Felipe Massa was only a tenth of a second back, with Sergio Perez 0.128secs adrift, but with Force India using last year's car as they wait for this season's challenger to be completed.

Behind Rosberg, McLaren and new power-unit partner Honda enjoyed their most productive pre-season day yet with Fernando Alonso completing 59 laps.

Neither McLaren nor Honda were anticipating much running after a seal broke on one of the two energy store systems on Thursday with Jenson Button at the wheel, and a replacement part not due until Saturday.

But McLaren managed to keep Alonso on track, with the Spaniard down by 1.387secs, although the team claim they are "50 per cent behind" on what they had planned for testing by this stage.

Britain's Jolyon Palmer, on his debut as Lotus reserve driver, was eighth quickest, but delighted to finally put in some laps.

Palmer, last year's GP2 series champion, said: "The day went well as we racked up a lot of laps and I got a good feel for the car.

"There was a lot to take on board with the different settings for the car and the different programmes we were running, so it's been a pretty steep learning curve.

"It's been a very positive start for me. I hope my work has benefited the team. I loved being in the car and I can't wait to get back out on track."

Sauber's Marcus Ericsson and rookie Carlos Sainz Jr brought up the rear, the latter over four seconds adrift, but completing 100 laps.