Veteran Michael Schumacher posted the fastest time in a rain-soaked first practice for the Belgian Grand Prix on Friday.
The seven-time Formula One champion, who made his debut at Spa 20 years ago, went out early when the track was still dry and before the rain starting lashing down.
The 42-year-old clocked a best time of 1 minute, 54.355 seconds, with his Mercedes teammate Nico Rosberg, who also went out early, second quickest in 1:54.829.
British driver Jenson Button was third fastest, but more than 8 seconds behind Schumacher.
There was a lull for some 20 minutes as the downpour was so heavy that most drivers - including defending F1 champion Sebastian Vettel, Button and his McLaren teammate Lewis Hamilton - preferred to wait for it to abate.
Vettel was fourth quickest, followed by Hamilton. Both trailed Schumacher's marker by more than nine seconds, although the times are of little consequence given the poor racing conditions faced by the drivers who started later.
The 7-kilometer (4.3-mile) Spa circuit is the longest and one of the most demanding tracks on the calendar along with Monaco's. Its famed corners, such as Eau Rouge and Blanchimont push drivers to the limit, and prompted caution in such slippery conditions.
Vettel tried to go out early on intermediate tires as his team warned him over the radio to be careful as there were "lots of rivers" on the track.
Former F1 champion Hamilton then went out for a brief run but, like Vettel, quickly came back into the pits after his car started slipping and sliding on the Blanchimont turn.
Other drivers experienced greater difficulties while staying out in the rain.
Brazilian driver Bruno Senna, making his F1 return with Renault as a replacement for Nick Heidfeld, had a miserable run that ended when he crashed into the barriers on the ninth turn. He was unhurt and his car was taken away.
Rookie driver Paul di Resta's Force India was also lifted away late in the session, after he slid sideways and slammed into the crash barriers on the same turn as Senna, prompting race director Charlie Whiting to go and inspect the turn.