Lewis Hamilton topped the rain-affected opening free practice session for the Chinese Grand Prix.
The McLaren driver set two fast laps at the end of the session to head Mercedes driver Nico Rosberg by a second at the top of the timesheets as a flurry of drivers set their best laps in the dying minutes of practice.
Hamilton, like team-mate Jenson Button, ran an upgrade package on his McLaren including modified front and rear wings and a new floor during the session - the team taking advantage of new-for-2012 regulations allowing sensor rigs to be run during Friday practice to measure the wake off the rear of the car.
Overnight rain meant that the 90-minute session started in damp conditions and during the half-an-hour most teams focused on installation laps and aerodynamic test runs.
Red Bull Racing's Mark Webber was the first driver to set a serious laptime, getting down to a 1m39.558s on the third flying lap of a six-lap run on Pirelli's medium compound rubber.
That lap was good enough to top the timesheets for much of the session as shortly after he had completed it, light rain made track conditions worse. By this stage, only nine drivers had completed serious runs and their laptimes were largely unchallenged until the track dried up enough for slicks to be run effectively in the final 10 minutes of the session.
Michael Schumacher had been in contention to top the timesheets after swapping fastest times with Webber with a few minutes to go, but he ended up third two-tenths behind his teammate.
Behind him, the two Saubers of Sergio Perez and Kamui Kobayashi both improved at the eleventh hour as the track conditions rapidly improved, demoting Webber to sixth.
The Australian was running Red Bull's latest-specification exhaust setup, while team-mate Sebastian Vettel, who ended up one place and two-tenths behind him, used the exhaust setup the team ran early in testing to compare the performance of the two designs.
Button ended up eight fastest, complaining of traffic late on, with Scuderia Toro Rosso pairing Daniel Ricciardo and Jean-Eric Vergne rounding out the top 10, like most of the field setting their fastest times in the final few minutes on prime rubber.
All of the top 16 drivers improved late on, with Lotus driver Romain Grosjean down in 17th place the best of the drivers whose laps in the early dry phase of the session were not improved upon.
Force India reserve driver Jules Bianchi made his grand prix weekend debut as third driver at Force India, ending the morning in 20th place, setting his best time on intermediates unlike teammate Nico Hulkenberg, who used medium slicks.
His opposite number at Caterham, Dutchman Giedo van der Garde, also made his first appearance, lapping around 1.5s off Heikki Kovalainen's pace.
Pos Driver Team Time Laps
1. Lewis Hamilton McLaren-Mercedes 1m37.106 7
2. Nico Rosberg Mercedes 1m38.116s + 1.010 14
3. Michael Schumacher Mercedes 1m38.316s + 1.210 14
4. Sergio Perez Sauber-Ferrari 1m38.584s + 1.478 13
5. Kamui Kobayashi Sauber-Ferrari 1m38.911s + 1.805 12
6. Mark Webber Red Bull-Renault 1m38.977s + 1.871 15
7. Sebastian Vettel Red Bull-Renault 1m39.198s + 2.092 12
8. Jenson Button McLaren-Mercedes 1m39.199s + 2.093 6
9. Daniel Ricciardo Toro Rosso-Ferrari 1m39.748s + 2.642 16
10. Jean-Eric Vergne Toro Rosso-Ferrari 1m39.768s + 2.662 14
11. Fernando Alonso Ferrari 1m40.056s + 2.950 14
12. Felipe Massa Ferrari 1m40.153s + 3.047 14
13. Valtteri Bottas Williams-Renault 1m40.298s + 3.192 8
14. Nico Hulkenberg Force India-Mercedes 1m40.328s + 3.222 13
15. Pastor Maldonado Williams-Renault 1m40.540s + 3.434 12
16. Heikki Kovalainen Caterham-Renault 1m41.071s + 3.965 14
17. Romain Grosjean Lotus-Renault 1m41.204s + 4.098 14
18. Timo Glock Marussia-Cosworth 1m42.330s + 5.224 14
19. Giedo van der Garde Caterham-Renault 1m42.521s + 5.415 11
20. Jules Bianchi Force India-Mercedes 1m44.118s + 7.012 8
21. Pedro de la Rosa HRT-Cosworth 1m44.227s + 7.121 10
22. Charles Pic Marussia-Cosworth 1m44.500s + 7.394 15
23. Narain Karthikeyan HRT-Cosworth 1m47.204s + 10.098 12
24. Kimi Raikkonen Lotus-Renault 1m50.465s + 13.359 11
All Timing Unofficial