Formula One championship leader Fernando Alonso posted the fastest time in Saturday's third practice session at the Belgian Grand Prix, with drivers benefiting from good conditions to frantically fine-tune their cars for qualifying.

Heavy rain marred Friday's two practice runs, restricting drivers to a handful of laps, so the emphasis was on cramming as much testing as possible into the final practice run before the afternoon's qualifying.

This time, the Spa circuit was splashed in bright sunshine and completely dry.

Alonso, who is chasing his third Formula One title, had a best lap time of 1 minute, 48.542 seconds in his Ferrari. Kimi Raikkonen of Finland, the 2007 F1 champion, clocked 1:48.683.

Mexico's Sergio Perez was third, while Mercedes driver Nico Rosberg pulled out after five laps.

Both Red Bulls took it easy, with Mark Webber finishing with the seventh fastest time, and defending F1 champion Sebastian Vettel the ninth best.

Webber, however, must contend with a five-place grid penalty for the second time in three races for an unscheduled gearbox change, and will start Sunday's race no higher than sixth on the grid.

With so little time to practice on Friday, some drivers found themselves at odds with their cars.

Raikkonen seemed troubled early on when he said over race radio that he was "learning nothing" from his Lotus, while Rosberg wobbled off the track and came to a stop before taking out his steering wheel.

Jenson Button, the 2009 F1 champion, complained to his McLaren team that "we've got to look at the balance," and British driver Paul di Resta struggled to keep his Force India under control, mentioning to his team how the car kept "dancing about a lot."

With Ferrari's car seemingly designed for straight-line speed, Alonso headed out on a fresh set of hard tires and set the best time with a few minutes left.

The Spaniard looks to be in good shape as he goes for his third pole position of the season.