Valtteri Bottas went quickest for Williams in a frenzied final few minutes of third practice for Formula 1's 2014 Belgian Grand Prix, outpacing Red Bull's Daniel Ricciardo by 0.268 seconds.
Mercedes driver Nico Rosberg was the best of the title contenders in third.
Many drivers only had the opportunity to run one or two flying laps on the soft Pirelli tyre at the end of the session, and were thwarted by traffic.
After overnight rain a dry line had already formed in some places, thanks to the preceding GP3 qualifying session, as the F1 cars took to the circuit for their installation laps.
With the track temperature still hovering around the 16C mark and the surface still mostly wet, there was little activity for the first 30 minutes as most teams - apart from the tail-enders - elected not to risk their machinery.
Thus Marussia annexed the top two positions for half the session as Jules Bianchi (2m01.823s), Max Chilton (2m05.120s) and Andre Lotterer (2m07.529s) each essayed a tentative lap before reporting back to the pits.
Esteban Gutierrez was the first driver out on slick tyres, half an hour into the session. He instantly took 1.581s out of Bianchi's time, then improved again by over four seconds as his medium-compound Pirellis came up to temperature, and this provided the cue for others to join the fray.
The track evolved quickly over the final 20 minutes as increased activity broadened the dry line, and Ferrari fans will have been heartened by the sight of Kimi Raikkonen bolting on a set of soft tyres and briefly topping the timesheets with five minutes to go.
Bottas, Ricciardo and Rosberg then went faster using the same tyre choice, but Lewis Hamilton and Fernando Alonso were unable to eclipse their team-mates and finished fifth and sixth fastest.
Sebastian Vettel finished the session outside the top 10 having done all his running on the medium compound.
He has reverted to an older engine after an ignition problem in one cylinder set in motion a failure of his nominated power unit on Friday.
Pos Driver Team/Car Time Gap Laps 1. Valtteri Bottas Williams-Mercedes 1m49.465s 12 2. Daniel Ricciardo Red Bull-Renault 1m49.733s +0.268s 9 3. Nico Rosberg Mercedes 1m49.739s +0.274s 13 4. Kimi Raikkonen Ferrari 1m49.817s +0.352s 9 5. Lewis Hamilton Mercedes 1m49.817s +0.352s 13 6. Fernando Alonso Ferrari 1m49.890s +0.425s 9 7. Daniil Kvyat Toro Rosso-Renault 1m49.893s +0.428s 11 8. Jenson Button McLaren-Mercedes 1m50.203s +0.738s 11 9. Felipe Massa Williams-Mercedes 1m50.423s +0.958s 11 10. Jean-Eric Vergne Toro Rosso-Renault 1m50.535s +1.070s 10 11. Sergio Perez Force India-Mercedes 1m50.592s +1.127s 12 12. Kevin Magnussen McLaren-Mercedes 1m50.748s +1.283s 11 13. Sebastian Vettel Red Bull-Renault 1m50.814s +1.349s 10 14. Nico Hulkenberg Force India-Mercedes 1m50.866s +1.401s 11 15. Adrian Sutil Sauber-Ferrari 1m50.962s +1.497s 12 16. Romain Grosjean Lotus-Renault 1m51.509s +2.044s 9 17. Pastor Maldonado Lotus-Renault 1m51.610s +2.145s 10 18. Esteban Gutierrez Sauber-Ferrari 1m51.898s +2.433s 15 19. Jules Bianchi Marussia-Ferrari 1m52.457s +2.992s 14 20. Max Chilton Marussia-Ferrari 1m52.984s +3.519s 14 21. Marcus Ericsson Caterham-Renault 1m54.294s +3.829s 11 22. Andre Lotterer Caterham-Renault 1m55.008s +5.543s 13