Fernando Alonso emerged quickest for Ferrari as Spa's famous changeable conditions dictated proceedings in Formula 1's first session following its summer hiatus.
Early rain over the Ardennes circuit meant the first and final sectors were saturated, in contrast to the middle split, where a dry line was rapidly established.
After the opening 15 minutes, in which McLaren's Jenson Button dominated, the mixed conditions effectively left the circuit too wet for slicks and too dry for intermediates, leading to a lull in activity.
The session finally sparked into life on the 45-minute mark as Mark Webber headed out on Pirelli's hard compound and immediately went fastest in sector two, even if his overall lap was some nine seconds off Button's early benchmark.
Webber's run, combined with several forecasts pointing to further showers in the closing minutes, inspired the entire the field to switch onto slicks and led to myriad changes at the top of the order.
Sergio Perez was the first man to displace team-mate Button with just over 30 minutes of the session to run, and on his next lap became the first man to dip below the 2m00s barrier.
Button reasserted himself immediately, only to be knocked off in turn by Nico Rosberg, Daniel Ricciardo and then Lewis Hamilton.
Ferrari was still yet to post a single flying effort as the session moved into the final half hour, but when it did emerge Alonso instantly hit the front, moving clear with a 1m57.1s.
There was still time for that to be usurped - firstly by Perez and then by Force India's Paul di Resta - but Alonso's next lap finally settled matters, earning top spot by just two hundredths of a second.
Di Resta's effort was enough for second, while his team-mate Adrian Sutil slipped into third late on, just ahead of Perez, Rosberg and Vettel.
Despite the conditions everyone managed to stay clear of the barriers, with incidents limited to a few trips down escape roads or run-off areas, particularly at La Source and the final chicane.
Pos Driver Team/Car Time Gap Laps 1. Fernando Alonso Ferrari 1m55.198s 11 2. Paul di Resta Force India-Mercedes 1m55.224s +0.026s 10 3. Adrian Sutil Force India-Mercedes 1m55.373s +0.175s 11 4. Sergio Perez McLaren-Mercedes 1m55.518s +0.320s 14 5. Nico Rosberg Mercedes 1m55.614s +0.416s 10 6. Sebastian Vettel Red Bull-Renault 1m55.636s +0.438s 14 7. Esteban Gutierrez Sauber-Ferrari 1m55.954s +0.756s 18 8. Nico Hulkenberg Sauber-Ferrari 1m56.110s +0.912s 11 9. Daniel Ricciardo Toro Rosso-Ferrari 1m56.770s +1.572s 14 10. Valtteri Bottas Williams-Renault 1m56.858s +1.660s 18 11. Felipe Massa Ferrari 1m56.863s +1.665s 10 12. Pastor Maldonado Williams-Renault 1m57.081s +1.883s 14 13. Jean-Eric Vergne Toro Rosso-Ferrari 1m57.084s +1.886s 17 14. Jenson Button McLaren-Mercedes 1m57.281s +2.083s 14 15. Lewis Hamilton Mercedes 1m57.358s +2.160s 10 16. Heikki Kovalainen Caterham-Renault 1m57.821s +2.623s 16 17. Giedo van der Garde Caterham-Renault 1m57.887s +2.689s 16 18. Max Chilton Marussia-Cosworth 1m58.600s +3.402s 14 19. Mark Webber Red Bull-Renault 1m58.929s +3.731s 12 20. Jules Bianchi Marussia-Cosworth 1m59.209s +4.011s 12 21. Kimi Raikkonen Lotus-Renault 1m59.441s +4.243s 11 22. Romain Grosjean Lotus-Renault 2m03.176s +7.978s 15