Lewis Hamilton grabbed the top spot from Fernando Alonso in the closing stages of first practice for the Korean Grand Prix.
Alonso's Ferrari had occupied first place for the majority of the session.
Ahead of a crucial race for his title hopes following his Suzuka first-lap exit, the Formula 1 championship leader put himself in front early on and then kept improving his time, eventually reaching a 1m39.450s.
With the field then switching to longer runs for the final half hour, it looked unlikely that Alonso would be beaten.
But four minutes from the end, Hamilton produced a 1m39.280s a long way into a stint to depose the Ferrari.
His strong pace continued, and as the chequered flag flew, Hamilton threw in a 1m39.148s on his 23rd lap of the run to give McLaren Friday morning honours by 0.302 seconds.
Red Bull's Mark Webber also made a late improvement, but it was not sufficient to get him above third.
His team-mate Sebastian Vettel was fifth, behind the second Ferrari of Felipe Massa. The Red Bulls both appeared stymied by heavy traffic at the end of the session - nearly tripping over each other at one point.
Mercedes duo Michael Schumacher and Nico Rosberg were next up, despite Schumacher heading to the garage for a while as he was unhappy with the feeling of the car.
While Romain Grosjean took eighth, his Lotus team-mate Kimi Raikkonen was 11th in the revised version of the car, featuring a Coanda-style exhaust among other updates.
The final top 10 spots went to Paul di Resta's Force India and Jenson Button's McLaren.
Jules Bianchi was the best of the Friday drivers in 13th place in Nico Hulkenberg's Force India.
Williams's Valtteri Bottas was back in 18th after a long spell in the pits having various major components changed to allow back to back tests with updated parts.
Giedo van der Garde returned to the Caterham cockpit and was 22nd despite a violent trip over a kerb.
Most drivers completed the session without drama, although Timo Glock was limited to 13 laps as his Marussia required attention to its steering.
Pos Driver Car Time Gap Laps
1. Lewis Hamilton McLaren-Mercedes 1m39.148s 23
2. Fernando Alonso Ferrari 1m39.450s + 0.302s 21
3. Mark Webber Red Bull-Renault 1m39.575s + 0.427s 21
4. Felipe Massa Ferrari 1m39.854s + 0.706s 23
5. Sebastian Vettel Red Bull-Renault 1m40.088s + 0.940s 21
6. Michael Schumacher Mercedes 1m40.221s + 1.073s 21
7. Nico Rosberg Mercedes 1m40.396s + 1.248s 24
8. Romain Grosjean Lotus-Renault 1m40.422s + 1.274s 22
9. Paul di Resta Force India-Mercedes 1m40.440s + 1.292s 19
10. Jenson Button McLaren-Mercedes 1m40.480s + 1.332s 21
11. Kimi Raikkonen Lotus-Renault 1m40.929s + 1.781s 14
12. Pastor Maldonado Williams-Renault 1m41.048s + 1.900s 25
13. Jules Bianchi Force India-Mercedes 1m41.140s + 1.992s 21
14. Kamui Kobayashi Sauber-Ferrari 1m41.220s + 2.072s 19
15. Sergio Perez Sauber-Ferrari 1m41.514s + 2.366s 20
16. Daniel Ricciardo Toro Rosso-Ferrari 1m41.596s + 2.448s 23
17. Jean-Eric Vergne Toro Rosso-Ferrari 1m42.021s + 2.873s 25
18. Valtteri Bottas Williams-Renault 1m42.027s + 2.879s 23
19. Heikki Kovalainen Caterham-Renault 1m42.104s + 2.956s 24
20. Timo Glock Marussia-Cosworth 1m42.175s + 3.027s 13
21. Charles Pic Marussia-Cosworth 1m42.706s + 3.558s 22
22. Giedo van der Garde Caterham-Renault 1m42.820s + 3.672s 19
23. Pedro de la Rosa HRT-Cosworth 1m44.517s + 5.369s 23
24. Dani Clos HRT-Cosworth 1m45.735s + 6.587s 22
All timing unofficial