Mercedes' Nico Rosberg top in first Monaco practice
Mercedes driver Nico Rosberg headed Ferrari's Fernando Alonso in first practice at the Monaco Grand Prix.
The German was just 0.087 seconds quicker than Alonso, who was on target to beat Rosberg's time before aborting his lap when he encountered traffic.
Lotus's Romain Grosjean was third from Ferrari's Felipe Massa and Mercedes' Lewis Hamilton, 0.274secs off Rosberg.
Williams's Pastor Maldonado was sixth, from Red Bull's Mark Webber and McLaren's Jenson Button.
World champion Sebastian Vettel was 10th in his Red Bull, behind McLaren's Sergio Perez, but ahead of Lotus's Kimi Raikkonen and Force India's Paul di Resta.
BBC 5 live analyst Allan McNish said the Red Bull drivers looked less comfortable out on the track than other leading contenders, with both Webber and Vettel struggling from locking brakes.
"I don't see the Red Bulls hooked up at this stage of the weekend," the Scot said.
BBC F1 pit-lane reporter Gary Anderson added that the Red Bulls' tyres were 'graining' - tearing on the surface - more than those of the Ferrari.
"We see the Mercedes are fast," McNish said. "Nico Rosberg is really on his game again and Lewis will have to step it up if he is to beat him here - but if anyone can do it, Hamilton can.
"Ferrari are strong - both Alonso and Massa are up there. Grosjean looked competitive but Raikkonen was a bit more scrappy. The surprise for me is Maldonado."
Maldonado's form was encouraging for Williams, who have had a troubled start to the season.
The Venezuelan was using the front wing and nose from the team's 2012 car - including a stepped top to the chassis rather than the smooth lines of the 2013 car - as Williams seek to turn around their form.
Despite the tight confines of the notoriously demanding Monaco street circuit, it was a largely incident-free session.
Force India's Adrian Sutil was extremely fortunate to avoid damaging his car when he spun at the first corner, Sainte Devote, and his rear wing just skimmed the barriers as the car rotated to a halt.
Webber also had an incident at Sainte Devote, taking to the run-off area, while Lotus's Romain Grosjean had a couple of off-track moments at the chicane as he built up to his fastest time.
The session took place under gloriously blue skies on the Cote d'Azur as Formula 1's blue riband event got off to a perfect start.
First Practice result:
1. Nico Rosberg - Mercedes - 1:16.195
2. Fernando Alonso - Ferrari - 1:16.282
3. Romain Grosjean - Lotus - 1:16.380
4. Felipe Massa - Ferrari -1:16.394
5. Lewis Hamilton - Mercedes - 1:16.469
6. Pastor Maldonado - Williams - 1:16.993
7. Mark Webber - Red Bull - 1:17.020
8. Jenson Button - McLaren - 1:17.129
9. Sergio Perez - McLaren - 1:17.378
10. Sebastian Vettel - Red Bull - 1:17.380
11. Kimi Raikkonen - Lotus - 1:17.509
12. Paul Di Resta - Force India - 1:17.548
13. Adrian Sutil - Force India - 1:17.625
14. Nico Hulkenberg - Sauber - 1:18.193
15. Jean-Eric Vergne - Toro Rosso - 1:18.454
16. Esteban Gutierrez - Sauber - 1:18.754
17. Valtteri Bottas - Williams - 1:18.830
18. Daniel Ricciardo - Toro Rosso - 1:19.067
19. Giedo van der Garde - Caterham - 1:19.203
20. Charles Pic - Caterham - 1:19.438
21. Jules Bianchi - Marussia - 1:19.773
22. Max Chilton - Marussia - 1:20.225
Monaco Grand Prix, day one
- Thursday, 23 May:
- Second practice: BBC Radio 5 live sports extra 13:00-14:30 BST, live text commentary on the BBC Sport website.
Monaco Grand Prix, day two
- Saturday, 25 May:
- Third practice: Live text and audio commentary on the BBC Sport website. Qualifying: BBC Radio 5 live 13:00, live text commentary on the BBC Sport website. TV highlights 17:35 BBC One
Monaco Grand Prix, day three
- Sunday, 26 May:
- Race: Coverage on BBC Radio 5 live and live text commentary on on the BBC Sport website. TV highlights 17:35 BBC One