Nico Rosberg finished the third day of testing at Barcelona with the fastest time for Mercedes GP.
The German's best lap of 1m20.686s from the morning, set before rain disrupted the action for a couple of hours, was enough to keep him at the top of the timesheets ahead of Sebastien Buemi's Toro Rosso.
Both drivers switched from short runs to long runs in the afternoon, and both suffered technical problems that forced them to stop on track. Rosberg stopped in the closing stages of a race simulation, while Buemi ran out of fuel a couple of laps into a run that came after his own race simulation had been completed.
The red flag for the Toro Rosso left the teams with less than 20 minutes to work with at the end of the day, and several of them opted to try some short runs, leading to plenty of changes to the leaderboard in the closing stages.
World champion Jenson Button jumped up to third late on, having spent most of his day working on race simulation in the McLaren.
Rubens Barrichello was another to focus on long runs in the afternoon, but he then improved to dip into the 1m21s at the end of the day, before the Williams brought an end to the day's action when it stopped just before Turn 7.
Felipe Massa also worked on long runs for most of the day, and the fifth-placed Ferrari driver's best time came in his final stint of a race simulation, shortly after he stopped for fresh tyres.
Some qualifying runs for Vitaly Petrov moved Renault closer to the pace than it has been so far this test in sixth, while Adrian Sutil was seventh for Force India.
Sebastian Vettel finished the day in eighth place having only notched up 51 laps by the end of proceedings. The Red Bull driver hardly ran at all in the morning, but did three runs of nine or 10 laps in the afternoon, with his final outing yielding his best time.
Jarno Trulli completed over 100 laps for Lotus on his way to ninth quickest, while Kamui Kobayashi's focus on long runs meant that the Sauber driver did not set any spectacular times and was classified 10th.
Virgin ended a difficult day with the slowest time, but after hardly setting a lap time all day Timo Glock at least managed to get close to the pace with a late run.
Pos Driver Team Time Gap Laps 1. Nico Rosberg Mercedes 1m20.686s 128 2. Sebastien Buemi Toro Rosso 1m21.413s + 0.727s 106 3. Jenson Button McLaren 1m21.450s + 0.764s 105 4. Rubens Barrichello Williams 1m21.975s + 1.289s 101 5. Felipe Massa Ferrari 1m22.344s + 1.658s 115 6. Vitaly Petrov Renault 1m22.523s + 1.837s 68 7. Adrian Sutil Force India 1m22.606s + 1.920s 61 8. Sebastian Vettel Red Bull 1m23.123s + 2.437s 51 9. Jarno Trulli Lotus 1m25.059s + 4.373s 102 10. Kamui Kobayashi Sauber 1m26.216s + 5.530s 105 11. Timo Glock Virgin 1m26.305s + 5.619s 31 All timing unofficial