Japanese driver Kamui Kobayashi was fastest in the rain-soaked first practice session at the Belgian Grand Prix on Friday, with championship leader Fernando Alonso preferring to stay in the Ferrari garage.

Defending champion Sebastian Vettel waited until near the end of the session before posting the ninth quickest time in the hard rain, but he was holding his Red Bull car back on the slippery circuit.

Kobayashi clocked 2 minutes, 11.389 seconds for Sauber, with Venezuelan rival Pastor Maldonado second quickest in 2:11.941 for Williams, followed by Daniel Ricciardo of Australia in his Toro Rosso.

Felipe Massa tried to come out in the closing stages but decided against it as smoke started to spew from the rear of his Ferrari.

More rain was expected for the second practice run.