Barcelona, May 11: McLaren's Jenson Button was fastest in practice for the Spanish Grand Prix, edging Formula One points leader Sebastian Vettel of Red Bull in a slippery final session on Friday.
The British driver's lap time of 1 minute, 23.399 seconds around the Catalunya Circuit was under two-tenths of a second quicker than two-time defending champion Vettel, who won the race last year.
Nico Rosberg of Mercedes, McLaren teammate Lewis Hamilton and Kimi Raikkonen of Lotus were the only other drivers within half a second of Button.
A number of drivers ran off track as they grappled with tires on the hot surface, with Red Bull's Mark Webber and Williams' Bruno Senna performing the most dramatic spins.
Webber, who went off wide at turn 4 to bring stones back onto the track from the run-off, finished behind Romain Grosjean of Lotus in seventh ahead of Michael Schumacher of Mercedes.
Schumacher had complained about the Pirelli tires being difficult to handle coming into the race as F1's hopefuls chase Vettel in what has started out as one of the most unpredictable seasons with four different race winners going into the first Europe-based GP of the season.
Most drivers switched to soft tires in the afternoon with the track temperature touching 45 degrees Celsius (113 F), and many struggled to stay on the circuit as the Pirellis quickly degraded.
Following the Bahrain GP, Vettel has 53 points to lead Hamilton by four points and Webber by five. Button and Ferrari's Fernando Alonso share 43 points.
Alonso's leading lap time in the opening session was his best of the two sessions, with the Spanish driver more than a second off Button's pace in front of his home fans. The two-time world champion's afternoon time left him 14th.
Alexander Rossi was the first American driver on F1's grid in five years as he tested for Caterham during the first session. Aside from swerving off track in one instance, the 20-year-old Rossi ran an error-free 25 laps, registering a best lap of 1:28.448.
"I enjoyed every minute of it," said Rossi, who normally competes in the World Series by Renault.
Spanish team HRT managed to get both of its Spanish drivers out on track in the morning as Pedro de la Rosa and Dani Clos filled out the bottom two of the 24-car field, with Clos making his 2012 debut.
Testing continues on Saturday ahead of qualifying for Sunday's 66-lap race.