Lewis Hamilton set the pace in opening practice for the Korean Grand Prix as Kimi Raikkonen suffered a big crash at the end of the session.
Mercedes driver Hamilton posted a lap of 1m39.630s to beat championship leader Sebastian Vettel by 0.037s, although the Red Bull driver had all the fastest sectors of the session.
Team-mate Mark Webber was third quickest and the only other man to lap in the 1m39s.
Nico Rosberg in the Mercedes was fourth ahead of McLaren's Jenson Button and Ferrari driver Fernando Alonso. Romain Grosjean was seventh ahead of Raikkonen, who ended the session with a heavy crash.
As the session was coming to a close, Raikkonen lost control of his Lotus and crashed heavily at the final corner, causing heavy damage to his car.
The Finn was able to walk away from the crash, but the team faces a race against the time to have the car ready for second practice.
Although perfect weather conditions welcomed the drivers at the start of the session, it took over 15 minutes before the first lap time was set by Rodolfo Gonzalez.
The Marussia driver had the track basically to himself for the first half hour, but that did not stop him from enduring drama, the Venezuelan losing control of his car and crashing against the barriers at Turn 13.
He lost his front wing and returned to the pits, leaving the circuit empty for several more minutes.
McLaren's Sergio Perez and Button joined Gonzalez in the timesheets with over 35 minutes of the session gone as the action finally began to pick up. Perez was the pacesetter for several minutes as most of the frontrunners continues to wait in their garages for the track conditions to improve.
Mercedes' Rosberg relegated Perez to second at the 50-minute mark, the German posting a 1m41.498s that was soon bettered by less than a tenth by Alonso.
Rosberg fought back seconds later to regain the top spot, the Mercedes driver followed by Vettel and Raikkonen, all of them lapping in the 1m40s.
Hamilton became the first man to lap in the 1m39s with half an hour left, the Briton outpacing his team-mate by nearly half a second as Webber jumped up to second behind the Briton.
Hamilton's time would remain unbeaten until the end, although Vettel moved up to second with a minute of the session left.
Apart from Gonzalez's and Raikkonen's crashes, the session included spins from James Calado and Hamilton, as well as several offs without further consequences.
Felipe Massa's session was curtailed by a puncture that meant the Brazilian spent most of the 90 minutes in the pits, the Ferrari driver completing his first timed lap with just 12 minutes to go.
Pos Driver Team Time Gap Laps 1. Lewis Hamilton Mercedes 1m39.630s 20
2. Sebastian Vettel Red Bull-Renault 1m39.667s +0.037s 20
3. Mark Webber Red Bull-Renault 1m39.816s +0.186s 19
4. Nico Rosberg Mercedes 1m40.117s +0.487s 20
5. Jenson Button McLaren-Mercedes 1m40.215s +0.585s 22
6. Fernando Alonso Ferrari 1m40.374s +0.744s 18
7. Romain Grosjean Lotus-Renault 1m40.396s +0.766s 19
8. Kimi Raikkonen Lotus-Renault 1m40.677s +1.047s 15
9. Sergio Perez McLaren-Mercedes 1m40.860s +1.230s 20
10. Felipe Massa Ferrari 1m40.880s +1.250s 13
11. Nico Hulkenberg Sauber-Ferrari 1m40.899s +1.269s 20
12. Adrian Sutil Force India-Mercedes 1m41.432s +1.802s 18
13. Pastor Maldonado Williams-Renault 1m41.482s +1.852s 22
14. Esteban Gutierrez Sauber-Ferrari 1m41.626s +1.996s 21
15. Jean-Eric Vergne Toro Rosso-Ferrari 1m41.924s +2.294s 19
16. Valtteri Bottas Williams-Renault 1m42.002s +2.372s 20
17. Daniel Ricciardo Toro Rosso-Ferrari 1m42.043s +2.413s 18
18. James Calado Force India-Mercedes 1m43.008s +3.378s 21
19. Charles Pic Caterham-Renault 1m43.660s +4.030s 20
20. Giedo van der Garde Caterham-Renault 1m43.883s +4.253s 19
21. Max Chilton Marussia-Cosworth 1m44.100s +4.470s 14
22. Rodolfo Gonzalez Marussia-Cosworth 1m46.810s +7.180s 10