Nico Rosberg narrowly led a close first free practice session for the Monaco Grand Prix on Thursday morning.
The Mercedes driver, who is hunting for a third straight pole position after topping qualifying in Bahrain and Spain, set a 1m16.195s.
That was enough to edge out Fernando Alonso's Ferrari by just 0.087 seconds.
The majority of the quick times came during a busy spell in the middle of the session. There was scant track action in the opening half-hour, but once the frontrunners came out, leading positions shuffled frequently for a brief period before the field settled into longer runs.
Felipe Massa had swapped the top spot with Rosberg a few times before the Mercedes and Alonso's sister Ferrari pushed the Brazilian down to third.
He then dropped another spot to fourth when Romain Grosjean put his Lotus ahead with one of the few late improvements.
Grosjean had earlier lost a little track time after a minor brush with the barrier meant his car needed attention in the pits.
Lewis Hamilton completed the top five in the second Mercedes.
Monaco specialist Pastor Maldonado is banking on this weekend giving Williams some much needed encouragement. Sixth in practice one was a good start.
The Red Bulls of Mark Webber and Sebastian Vettel were back in seventh and 10th, split by McLaren duo Jenson Button and Sergio Perez.
The only significant incident of the session came when Adrian Sutil spun and gave his Force India a knock at Sainte Devote in the closing minutes.
Hamilton had earlier skated down the escape road at the same point.
Pos Driver Team Time Gap Laps
1. Nico Rosberg Mercedes 1m16.195s 30
2. Fernando Alonso Ferrari 1m16.282s + 0.087s 26
3. Romain Grosjean Lotus-Renault 1m16.380s + 0.185s 20
4. Felipe Massa Ferrari 1m16.394s + 0.199s 22
5. Lewis Hamilton Mercedes 1m16.469s + 0.274s 27
6. Pastor Maldonado Williams-Renault 1m16.993s + 0.798s 26
7. Mark Webber Red Bull-Renault 1m17.020s + 0.825s 26
8. Jenson Button McLaren-Mercedes 1m17.129s + 0.934s 28
9. Sergio Perez McLaren-Mercedes 1m17.378s + 1.183s 24
10. Sebastian Vettel Red Bull-Renault 1m17.380s + 1.185s 22
11. Kimi Raikkonen Lotus-Renault 1m17.509s + 1.314s 25
12. Paul di Resta Force India-Mercedes 1m17.548s + 1.353s 26
13. Adrian Sutil Force India-Mercedes 1m17.625s + 1.430s 20
14. Nico Hulkenberg Sauber-Ferrari 1m18.193s + 1.998s 25
15. Jean-Eric Vergne Toro Rosso-Ferrari 1m18.454s + 2.259s 24
16. Esteban Gutierrez Sauber-Ferrari 1m18.754s + 2.559s 27
17. Valtteri Bottas Williams-Renault 1m18.830s + 2.635s 27
18. Daniel Ricciardo Toro Rosso-Ferrari 1m19.067s + 2.872s 24
19. Giedo van der Garde Caterham-Renault 1m19.203s + 3.008s 20
20. Charles Pic Caterham-Renault 1m19.438s + 3.243s 27
21. Jules Bianchi Marussia-Cosworth 1m19.773s + 3.578s 19
22. Max Chilton Marussia-Cosworth 1m20.225s + 4.030s 20