Nico Rosberg headed Lewis Hamilton to a Mercedes one-two in first practice at the Chinese Grand Prix.
Rosberg was nearly half a second faster than Hamilton, with Red Bull's Mark Webber third a similar margin behind.
Webber's team-mate Sebastian Vettel was fourth, ahead of Ferrari's Fernando Alonso and McLaren's Jenson Button.
Ferrari's Felipe Massa was seventh, ahead of Force India's Adrian Sutil, with Lotus's Romain Grosjean separating Sutil from team-mate Paul di Resta.
Grosjean's team-mate Kimi Raikkonen was 11th fastest after a session that included a quick spin for the Finn at Turn Nine.
Rosberg, who dominated this race last year for Mercedes' only win of the season, was in strong form from the start of his running, a similar margin ahead of Hamilton on both of his fastest runs.
His fastest time of one minute 36.717 seconds was 0.454secs quicker than Hamilton could manage.
The Englishman missed Thursday's pre-event interviews with an allergic reaction that was causing him some eye problems.
"That's a very good run from Mercedes," said BBC F1 technical analyst Gary Anderson.
With all eyes on Red Bull following the team orders controversy in Malaysia, Webber had a margin on Vettel during the session, ending up 0.284secs faster than the world champion.
Alonso was just 0.023secs behind Vettel and 0.13secs faster than Massa, who has the chance this weekend to become the first team-mate ever to out-qualify Alonso for five races in a row, including the end of last season.
Button and team-mate Sergio Perez, who was 16th fastest, were assessing some redesigned bodywork on the McLaren, introduced in an attempt to solve some of the aerodynamic problems that have blighted their start to the season.
Perez ended the session in the tyre wall at the entry to the pits, after losing control in the pit lane.
BBC F1 technical analyst Gary Anderson said the aggressive sweep of the rear bodywork had been tamed in an attempt to solve the aerodynamic problems the team have been experiencing since the start of the season.
Most of the teams had upgrades on the cars following the three-week break since Malaysia as they try to either eke out extra performance or, in the case of teams such as struggling McLaren and Williams, solve problems that emerged at the start of the season.
First Practice result:
1. Nico Rosberg - Mercedes - 1:36.717
2. Lewis Hamilton - Mercedes - 1:37.171
3. Mark Webber - Red Bull - 1:37.658
4. Sebastian Vettel - Red Bull - 1:37.942
5. Fernando Alonso - Ferrari - 1:37.965
6. Jenson Button - McLaren - 1:38.069
7. Felipe Massa - Ferrari - 1:38.095
8. Adrian Sutil - Force India - 1:38.125
9. Romain Grosjean - Lotus - 1:38.398
10. Paul di Resta - Force India - 1:38.561
11. Kimi Raikkonen - Lotus - 1:38.790
12. Jean-Eric Vergne - Toro Rosso - 1:39.057
13. Pastor Maldonado - Williams - 1:39.158
14. Nico Hulkenberg - Sauber - 1:39.180
15. Daniel Ricciardo - Toro Rosso - 1:39.336
16. Sergio Perez - McLaren - 1:39.360
17. Valtteri Bottas - Williams - 1:39.392
18. Esteban Gutierrez - Sauber - 1:40.032
19. Jules Bianchi - Marussia - 1:41.966
20. Max Chilton - Marussia - 1:42.056
21. Giedo van der Garde - Caterham - 1:42.083
22. Qinghua Ma - Caterham - 1:43.545