Mark Webber went quickest in a very wet opening practice session for this weekend's British Grand Prix.
The Australian's best of 1m46.603s came late in the session and it was enough to outpace Mercedes' Michael Schumacher by over half a second on a morning that gave no answers as to how the cars have been affected by the ban on off-throttle diffusers.
On the wet track, however, things did not seem to change much, as Webber kept Red Bull ahead of its rivals once more.
Rubens Barrichello posted the third quickest time for Williams, with Sauber's Sergio Perez completing a promising session by finishing as fourth fastest. Felipe Massa was the quickest of the Ferraris, the Brazilian completing the top five.
Virgin's Jerome d'Ambrosio was the first man to complete a timed lap nearly 20 minutes into the session, with Nico Rosberg next to set a time, going 11 seconds quicker than the Belgian.
Button moved to the top of the times 26 minutes into the session, while Alonso, using Ferrari's new rear wing, spun without consequence during his first timed run.
Webber was next to go quickest, the Australian stopping the clock at 1m50.032s on intermediate tires. Button regained first place moments later with the first lap in the 1m49s as the action on track intensified, with teammate Hamilton taking the top spot with 57 minutes left, only for Button to outpace him right away.
As the times continued to improved, it was Vettel who jumped to first with some 50 minutes to go, the World Champion lapping in the 1m48s. Despite a trip through the gravel, Schumacher outpaced Vettel thee minutes later to move to the top of the times.
Schumacher stayed in first place for several minutes, before Webber improved by less than a tenth to relegate the German to second as the threat of rain intensified. The Australian did not last long there, though, as Alonso beat his time by 0.024sec just a moment before rain hit the track with some 25 minutes to go.
The biggest incident of the day came when Kamui Kobayashi lost control of his car at Turn 18, the Japanese spinning on the wet curbs with 13 minutes left. His car came close to rolling, but luckily for Kobayashi it did not, and instead hit the barriers hard sideways. The Sauber driver managed to climb out unassisted, but his car was heavily damaged and his team faces a race against time to get it ready for the afternoon's session.
Track conditions improved slightly in the last 10 minutes and Webber made good use of that to jump to the top, becoming the first man to lap in the 1m47s and minutes later in the 1m46s.
Pos Driver Team Time Laps
1. Mark Webber Red Bull-Renault 1m46.603s 19
2. Michael Schumacher Mercedes 1m47.263s + 0.660 20
3. Rubens Barrichello Williams-Cosworth 1m47.347s + 0.744 23
4. Sergio Perez Sauber-Ferrari 1m47.422s + 0.819 22
5. Felipe Massa Ferrari 1m47.562s + 0.959 13
6. Nico Rosberg Mercedes 1m47.758s + 1.155 23
7. Fernando Alonso Ferrari 1m48.161s + 1.558 16
8. Lewis Hamilton McLaren-Mercedes 1m48.549s + 1.946 21
9. Nico Hulkenberg Force India-Mercedes 1m48.598s + 1.995 19
10. Jaime Alguersuari Toro Rosso-Ferrari 1m48.678s + 2.075 22
11. Paul di Resta Force India-Mercedes 1m48.730s + 2.127 18
12. Sebastien Buemi Toro Rosso-Ferrari 1m48.778s + 2.175 18
13. Sebastian Vettel Red Bull-Renault 1m48.794s + 2.191 21
14. Pastor Maldonado Williams-Cosworth 1m48.809s + 2.206 17
15. Jenson Button McLaren-Mercedes 1m48.841s + 2.238 23
16. Nick Heidfeld Renault 1m48.941s + 2.338 20
17. Vitaly Petrov Renault 1m49.603s + 3.000 15
18. Kamui Kobayashi Sauber-Ferrari 1m50.133s + 3.530 17
19. Jarno Trulli Lotus-Renault 1m50.222s + 3.619 14
20. Karun Chandhok Lotus-Renault 1m51.119s + 4.516 17
21. Timo Glock Virgin-Cosworth 1m52.470s + 5.867 17
22. Tonio Liuzzi HRT-Cosworth 1m53.143s + 6.540 20
23. Jerome D'Ambrosio Virgin-Cosworth 1m53.469s + 6.866 26
24. Daniel Ricciardo HRT-Cosworth 1m54.334s + 7.731 24
All Timing Unofficial