MONTMELO, Spain (AP) Nico Rosberg pushed his Mercedes to the fastest lap so far in preseason testing at the Barcelona-Catalunya circuit as Jenson Button saw his promising drive end in the grass on Friday.
Rosberg let loose a late flying lap of 1 minute, 22.792 seconds to put champions Mercedes on the top of the timesheet for the first time through 10 days of testing.
Rosberg's leading effort was over 2.5 seconds faster than teammate and champion Lewis Hamilton's pole-setting pace from the 2014 Spanish Grand Prix held at the same track.
''We have shown today that we are fast, but I don't know how we stack up to the other teams,'' said the cautious Rosberg. ''We have to wait until tomorrow and the next day and then we still won't know until Melbourne.''
McLaren has been plagued by a myriad of mechanical problems and is without Fernando Alonso for this last week of preparation as he recovers from Sunday's crash.
But after an overnight engine change by new supplier Honda, Button was 20 minutes away from a trouble-free day when he went off the track on his 101st lap due to what the team called an ''unspecified electrical issue.'' Button's tally was still McLaren's highest this winter. Button had previously not put in more than 35 on a given day, while Alonso's highest tally was 59.
''We've still got a lot of work still do to, but we had a good day today,'' Button said. ''It's never easy when you've missed out on so much testing, but we're making good progress, and we got lots of really useful information today.
''We know we're not the quickest guys out there, but you'll certainly see a lot of improvement by the time we get to Australia.''
Williams continued to impress as Valtteri Bottas clocked the second best lap time, a day after teammate Felipe Massa had gone the fastest.
Sauber newcomer Nasr Felipe also had another strong showing with the third quickest time.
Sebastian Vettel in his Ferrari was next, followed by Button in fifth.
McLaren wasn't the only team to make good use of the sunny and rain-free conditions for a very productive day of driving. Six of the nine drivers got in over 100 laps, with Vettel leading the pack with 143.
Force India's Nico Hulkenberg still managed 77 laps after finally debuting the team's new car just before the lunch break. Force India had missed the first test and used a 2014 car at the second test.
Toro Rosso's 17-year-old driver Max Verstappen had electrical issues with less than an hour to go when he was then leading the rest with 139 laps done.
The day was smooth going until the finally half hour, when Button went off between curves seven and eight. Once his car was cleared away, Pastor Maldonado of Lotus went off course- apparently with damage to his front wing- to bring the session to close under a red flag.
Testing concludes Sunday. The season starts at the Australian GP on March 15.