His team-mate, Felipe Massa, was second, with Toro Rosso's Jean-Eric Vergne in third.
The session started with drivers requiring to use full wet tyres. This was bad news for most teams, who are desperate to test their large update packages ahead of the race.
Romain Grosjean suffered a minor spin under braking at Turn 10 in his Lotus, while a few other drivers struggled while turning into the chicane.
The circuit dried sufficiently for slick tyres to be fitted for the final 15 minutes of the hour and a half session.
Conditions were still damp and they caught out Massa, who bounced through the gravel at Turn 2.
Grosjean recovered from his spin to set the fourth fastest time, ahead of Adrian Sutil and Lewis Hamilton. The Mercedes driver was on course for a top time but was held up during a busy final 10 minutes to the session.
Finns Valtteri Bottas and Kimi Raikkonen were seventh and eight, with Nico Rosberg and Paul di Resta rounding out the top 10.
McLaren broke the curfew on Thursday night to work on their large upgrade package, but Jenson Button was the only driver not to set a time with Sergio Perez also undertaking limited running.