Formula 1 rivals say Mercedes 'unbelievably quick' in 2015 testing

The German clocked a 1m22.792 on the soft tyres, which was 0.7 seconds quicker than the previous best set by Williams's Felipe Massa on the same tyre on Thursday.

It was also 1.2s faster than Massa's team-mate Valtteri Bottas managed as he took second to Rosberg on Friday at the Circuit de Catalunya.

When asked about Rosberg's lap time, Bottas said: "It's pretty quick. I think everyone was a little bit surprised. Everyone knew they were quick, but they were very quick.

"I don't know how much fuel they run or how much they took out of the engine but for sure they are still ahead. That's the fact at this point of the season."

Jenson Button, who completed more than 100 laps for what was McLaren's strongest day, added: "They are unbelievably quick and we are obviously a long way off, but we don't know what fuel everyone is running.

"The only thing you can look at are the long runs, which are very, very interesting. Mercedes is miles in front of anyone on long runs but some of the cars do have degradation."

Ferrari driver Sebastian Vettel, who finished fourth fastest, 2.5s off the pace, was also in awe of Rosberg's pace.

"I don't know which tyres he had, but I think it was a very strong lap time."

When he was told Mercedes had used the soft tyre, the four-time world champion added: "I think it is clear that if you want to win you need to beat Mercedes."