Mercedes gave their rivals a morale-sapping glimpse of their true pace in Formula One testing on Friday with Nico Rosberg lapping more than a second quicker than anyone else.

After days spent pounding around on long runs with the slower medium tyres, the champions changed their focus and sent Rosberg out on softs for the first time in this year's winter testing.

The result on a sunny afternoon was both quick and crushing, with the German 1.203 seconds quicker than second-placed Finn Valtteri Bottas had managed earlier on the softs in the Mercedes-powered Williams.

The 2014 runner-up's lap time of one minute 22.792 seconds was also seven-tenths faster than Brazilian Felipe Massa managed for Williams on Thursday although the track got quicker through use.

Rosberg managed a respectable 106 laps, with Ferrari's Sebastian Vettel taking the honours for the most distance covered with a hefty 143.

"They are unbelievably quick," McLaren's Jenson Button, himself buoyed by completing 101 laps for his struggling team despite stopping on the track towards the end, told reporters when asked about the Mercedes pace.

Rosberg, who hands over to world champion Lewis Hamilton on Saturday, played down the car's performance however.

"Today was a difficult day to start with but ended on a positive," he said.

"During the early runs, even though the numbers looked like they were in the right place with the setup, it was actually very far away from where it needed to be, which was unusual and caught us off guard a little bit.

"But eventually, towards the end especially, it was feeling more and more decent. Then, putting the soft tyres on, that gives you the grip anyway so it doesn't really matter what the balance is," added Rosberg.

Force India finally got their new car out on the track, with only two days of testing left after Friday before the freight has to be sent to Australia, but Nico Hulkenberg said they had their work cut out.

"We did 77 laps pretty much only in an afternoon, I think that's quite remarkable. The boys have done a good job there, pushing really hard over the last couple of weeks," said the German.

"Obviously we are playing catch up at the moment, doing the checks that everyone has done weeks ago. But as far as I can see it was a positive day."