Pastor Maldonado put Williams on top of the times in the opening practice session for the European Grand Prix in Valencia.
The Williams driver, the winner of the Spanish Grand Prix earlier this year, posted a best time of 1m40.890s early in the session and that benchmark remained unbeaten until the end.
Red Bull's Sebastian Vettel got closest to the Venezuelan, finishing just 0.083 seconds behind, with team-mate Mark Webber only a hundredth of a second adrift.
Jenson Button seemed to have a much better start to the weekend than in Canada after posting the fourth fastest time in the quickest of the McLarens, team-mate Lewis Hamilton was down in eighth position.
Fernando Alonso completed the top five in the Ferrari in a session in which the top nine drivers were covered by less than three tenths of a second.
Toro Rosso drivers Jean-Eric Vergne and Daniel Ricciardo were the only men to set a time in the first 30 minutes of the session, as the majority of their rivals took it easy after completing their installation laps.
Raikkonen was the first of the frontrunners to complete a timed lap with 55 minutes to go, the Finn posting a 1m42.589s, which was bettered a few moments later by Lotus team-mate Grosjean. Raikkonen regained the top spot with a second flyer, however, going into the 1m41s.
With 50 minutes left, it was Rosberg who jumped to first place in the Mercedes, going over half a second quicker than Raikkonen.
Maldonado in the Williams was first to lap in the 1m40s around the 50-minute mark, with local hero Alonso moving to second when he began to complete timed laps after several installation rounds.
World champion Vettel moved to second place with less than 10 minutes to go, as Venezuelan Maldonado kept the top spot in a trouble-free session.
Pos Driver Team Time Laps
1. Pastor Maldonado Williams-Renault 1m40.890s 22
2. Sebastian Vettel Red Bull-Renault 1m40.973s + 0.083 21
3. Mark Webber Red Bull-Renault 1m40.984s + 0.094 19
4. Jenson Button McLaren-Mercedes 1m40.994s + 0.104 19
5. Fernando Alonso Ferrari 1m41.065s + 0.175 26
6. Paul di Resta Force India-Mercedes 1m41.105s + 0.215 15
7. Michael Schumacher Mercedes 1m41.117s + 0.227 22
8. Lewis Hamilton McLaren-Mercedes 1m41.158s + 0.268 18
9. Nico Rosberg Mercedes 1m41.182s + 0.292 21
10. Kimi Raikkonen Lotus-Renault 1m41.620s + 0.730 21
11. Romain Grosjean Lotus-Renault 1m41.784s + 0.894 15
12. Kamui Kobayashi Sauber-Ferrari 1m41.838s + 0.948 19
13. Sergio Perez Sauber-Ferrari 1m41.861s + 0.971 16
14. Felipe Massa Ferrari 1m42.109s + 1.219 20
15. Jules Bianchi Force India-Mercedes 1m42.175s + 1.285 21
16. Valtteri Bottas Williams-Renault 1m42.299s + 1.409 24
17. Heikki Kovalainen Caterham-Renault 1m42.442s + 1.552 26
18. Jean-Eric Vergne Toro Rosso-Ferrari 1m42.758s + 1.868 26
19. Daniel Ricciardo Toro Rosso-Ferrari 1m42.777s + 1.887 28
20. Vitaly Petrov Caterham-Renault 1m43.209s + 2.319 19
21. Charles Pic Marussia-Cosworth 1m44.173s + 3.283 18
22. Pedro de la Rosa HRT-Cosworth 1m44.996s + 4.106 15
23. Narain Karthikeyan HRT-Cosworth 1m45.120s + 4.230 23
24. Timo Glock Marussia-Cosworth 1m45.338s + 4.448 7
All Timing Unofficial