Pastor Maldonado topped the third morning of testing in Barcelona, setting the fastest time of the week so far.
The Williams driver set the time on super-soft tires with half an hour of the morning session remaining while on a run of three flying laps.
The Venezuelan, who is having his first day in the car this week after Bruno Senna and Valtteri Bottas completed a day each, ended up almost one second clear of the second fastest driver, Michael Schumacher. The seven-time champion, driving the new Mercedes W03, set his best time on soft-compound Pirelli rubber only a few minutes before Maldonado went fastest.
Sauber driver Kamui Kobayashi was third fastest. He set his time with one hour of the morning remaining, like Schumacher running the soft compound but he later brought the session to a premature end moments before the checkered flag was due to fly after stopping at the exit of Turn 4 on an out-of-fuel run.
McLaren driver Jenson Button was fourth fastest in the MP4-27, lapping 1.5sec off Maldonado. He set his time shortly before midday, running soft rubber.
Scuderia Toro Rosso's Jean-Eric Vergne had his first taste of test action for this week, ending up fifth fastest, two seconds off the pace, and completed 42 laps of running. He finished ahead of Felipe Massa, who endured another busy morning of work as Ferrari attempts to get on top of its troubled machine. His program included a number of straightline aero runs on start/finish.
Red Bull had a quiet morning, with Mark Webber completing only 26 laps after taking over from Sebastian Vettel.
Timo Glock drove the Virgin MVR-02 for the first time since last year's Brazilian Grand Prix. He focused on tire evaluation work using each of the four available Pirelli compounds. Glock also triggered a red flag when he spun, but was able to make it back to the pits.
Caterham driver Vitaly Petrov ended the morning ninth fastest after suspension problems restricted his running. He ended up 0.25sec faster that Paul di Resta, whose Force India caused the red flag to be deployed very briefly when he lost part of his engine cover.
Pos Driver Team Time Laps
1. Maldonado Williams 1m22.391s 52
2. Schumacher Mercedes 1m23.384s +0.993 54
3. Kobayashi Sauber 1m23.582s +1.191 59
4. Button McLaren 1m23.918s +1.527 39
5. Vergne Toro Rosso 1m24.433s +2.042 42
6. Massa Ferrari 1m24.771s +2.380 48
7. Webber Red Bull 1m24.835s +2.444 26
8. Glock Marussia 1m26.173s +3.782 51
9. Petrov Caterham 1m26.929s +4.538 23
10. Di Resta Force India 1m27.180s +4.789 20