Jenson Button put McLaren on top of the times after the opening morning of Formula 1's final preseason test at Barcelona.
Button barely completed a lap during the first hour of running as he and McLaren practiced pit stops. But he was immediately quick after taking to the track and set his best lap of 1m23.510s on hard tires shortly before 11:30.
That was enough to knock Sergio Perez off the top spot by 0.31sec, and with nobody else improving over the remaining time, the Sauber driver remained second. Perez's best time came on medium tires, but he had earlier lapped on prototype medium rubber from Pirelli.
Romain Grosjean spent almost an hour in the pits early on, Lotus having to change sensor equipment on his E20 after the Frenchman's initial runs. He gradually moved up the order throughout the morning and went third with half an hour to go.
Mercedes driver Nico Rosberg, who completed numerous stints of eight laps each, was fourth, just 0.001sec slower than Grosjean while Mark Webber's Red Bull and Jean-Eric Vergne's Toro Rosso were next up, ahead of Force India driver Paul di Resta and Ferrari's Felipe Massa.
Vitaly Petrov, who was not scheduled to drive until Saturday, was a late call-up for Caterham after Heikki Kovalainen suffered food poisoning overnight. The Russian completed a session-high 67 laps, the team confirming that it had made adjustments to the brake blanking and front wing flaps late in the morning. He was an encouraging 1.4sec off Button's time and 0.7sec up on Pastor Maldonado's Williams, which was slowest.
Pos Driver Team Time Laps
1. Button McLaren 1m23.510s 27
2. Perez Sauber 1m23.820s +0.310 57
3. Grosjean Lotus 1m23.959s +0.449 36
4. Rosberg Mercedes 1m23.992s +0.482 57
5. Webber Red Bull 1m23.993s +0.483 53
6. Vergne Toro Rosso 1m24.216s +0.706 61
7. Di Resta Force India 1m24.305s +0.795 48
8. Massa Ferrari 1m24.318s +0.808 45
9. Petrov Caterham 1m24.876s +1.366 67
10. Maldonado Williams 1m25.587s +2.077 37