Jenson Button edges Sebastian Vettel

BARCELONA, Spain -- McLaren driver Jenson Button was fastest in practice for the Spanish Grand Prix, edging out Formula One leader Sebastian Vettel of Red Bull in a slippery final session Friday that indicated tires will be a big factor in the race.

The Briton's lap time of 1 minute, 23.399 seconds around Catalunya Circuit was less than two-tenths of a second quicker than Vettel, the two-time defending champion who won the race last year.

Teammate Lewis Hamilton, Nico Rosberg of Mercedes and Kimi Raikkonen of Lotus were the only other drivers within half a second of Button.

A number of others drove off the track after having to deal with a strong wind and tires disintegrating on the hot surface.

Red Bull's Mark Webber and Williams' Bruno Senna performed the most dramatic spins. Webber, who went off wide at Turn 4 to flick some stones on the track, finished behind Romain Grosjean of Lotus and just in front of Michael Schumacher of Mercedes in seventh place.

"You always want to try and get through all three sessions with minimal damage on your tires, and there will be no exception this weekend," Webber said. "We could see people sit out (the final qualifying session), those that just get into Q3, and they will do that to save tires."

Schumacher had complained about the Pirelli tires being difficult to handle coming into the race. Most drivers switched to soft tires in the afternoon with the track temperature hitting 113 degrees, and many struggled to stay on the circuit when tires quickly degraded.

"It has been a very challenging day, first of all to see how to get the car right for qualifying and especially how to get it right for the race," said Rosberg, who won in Shanghai. "I don't know where we are on tire wear. Tire management is going to be crucial here."

Following the Bahrain GP, Vettel has 53 points to lead Hamilton by four and Webber by five. Button and Ferrari's Fernando Alonso both have 43 points.

Alonso's leading lap time in the opening practice was his fastest of the two sessions, with the Spanish driver more than a second off Button's pace in front of his home fans. The former two-time world champion's afternoon time left him 14th.

"It's too early for us to say today how we will be in qualifying tomorrow but there's a good feeling at the moment," Alonso said. "Tomorrow will be the moment of truth."

Teams have also brought aerodynamic upgrades to the track following last week's testing session in Italy, and only one second separated the first 13 drivers in the afternoon.

"We've been fast everywhere else so far and there's nothing to suggest we won't be fast here again," said Raikkonen, a two-time winner here who is coming off a second-place finish in Bahrain.

Alexander Rossi was the first American driver on F1's grid in five years, testing 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.

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.

There's more testing Saturday ahead of qualifying for the 66-lap race.