Kamui Kobayashi emerged quickest for Sauber in a wet first practice session for the Belgian Grand Prix at Spa-Francorchamps.
Consistently heavy rain over the Ardennes circuit left the track saturated, resulting in a morning of little action.
Indeed in the first half of the session, Kobayashi was the only man willing to try a flying lap.
His 2m17.705s from an early four-lap run held a lonely top spot as all the other drivers settled for cautious installation runs.
Although the rain barely relented, more drivers took to the track going into the final 40 minutes.
Toro Rosso duo Daniel Ricciardo and Jean-Eric Vergne led the way for a while, before a busy five minutes around the hour mark featured Vergne, Kobayashi, Ricciardo and finally Williams driver Pastor Maldonado taking turns on top.
Kobayashi then reasserted himself with a 2m11.389s, 17 minutes from the end. That put the Sauber on top by 0.552 seconds over Maldonado, with Ricciardo and Vergne next up.
Red Bull's Mark Webber was the best of the title contenders in fifth place.
His team-mate Sebastian Vettel took ninth, behind Sergio Perez (Sauber), Nico Rosberg (Mercedes) and Williams reserve Valtteri Bottas.
Michael Schumacher escaped a scary moment at Eau Rouge to complete the top 10 with a late lap.
Points leader Fernando Alonso was among those choosing not to set a proper time, along with Ferrari team-mate Felipe Massa and the Lotus duo.
Massa also stopped at the chicane with smoke eminating from the rear of his Ferrari, apparently in some kind of engine trouble.
Despite the appalling conditions, everyone managed to stay clear of the barriers, with incidents limited to a few trips down escape roads and twitches of aquaplaning.
Pos Driver Car Time Gap Laps 1. Kamui Kobayashi Sauber-Ferrari 2m11.389s 20 2. Pastor Maldonado Williams-Renault 2m11.941s + 0.552s 14 3. Daniel Ricciardo Toro Rosso-Ferrari 2m12.004s + 0.615s 12 4. Jean-Eric Vergne Toro Rosso-Ferrari 2m12.824s + 1.435s 15 5. Mark Webber Red Bull-Renault 2m13.191s + 1.802s 13 6. Sergio Perez Sauber-Ferrari 2m13.861s + 2.472s 16 7. Nico Rosberg Mercedes 2m14.210s + 2.821s 14 8. Valtteri Bottas Williams-Renault 2m14.660s + 3.271s 16 9. Sebastian Vettel Red Bull-Renault 2m14.860s + 3.471s 12 10. Michael Schumacher Mercedes 2m15.402s + 4.013s 13 11. Paul di Resta Force India-Mercedes 2m15.812s + 4.423s 11 12. Timo Glock Marussia-Cosworth 2m16.409s + 5.020s 16 13. Nico Hulkenberg Force India-Mercedes 2m16.786s + 5.397s 10 14. Vitaly Petrov Caterham-Renault 2m16.788s + 5.399s 16 15. Lewis Hamilton McLaren-Mercedes 2m16.827s + 5.438s 5 16. Jenson Button McLaren-Mercedes 2m16.861s + 5.472s 8 17. Charles Pic Marussia-Cosworth 2m17.519s + 6.130s 14 18. Heikki Kovalainen Caterham-Renault 2m18.199s + 6.810s 10 19. Pedro de la Rosa HRT-Cosworth 2m19.546s + 8.157s 12 20. Dani Clos HRT-Cosworth 2m19.689s + 8.300s 12 21. Romain Grosjean Lotus-Renault 2m38.701s + 27.312s 9 22. Fernando Alonso Ferrari 2m40.749s + 29.360s 4 23. Kimi Raikkonen Lotus-Renault 2m46.580s + 35.191s 9 24. Felipe Massa Ferrari 2 All timing unofficial