Romain Grosjean topped GP2 free practice at Monaco by more than one second over the rest of the field.
The DAMS driver was the pacesetter for most of the session, as he was one of the few drivers to get a series of clear laps in. Grosjean was already out front by the halfway stage and, with seven minutes remaining, he found another half a second to put himself 0.8sec clear of Racing Engineering's Dani Clos at that time.
As the session came to a close, Grosjean put in another hot lap, where he was able to stretch his margin over the Spaniard to 1.1sec as the checkered flag fell.
Clos was followed up by the spectacular Stefano Coletti (Trident) and Luca Filippi (Super Nova), who both looked close to the edge on their best laps. Filippi's pressing eventually caught him out, as he spun at Massenet with three minutes to go, bringing his session to a premature end.
Davide Valsecchi was fifth fastest, but he was another to hit trouble. The AirAsia driver lost his rear wing when he spun on the entry to the final corner, hitting the barriers on the outside.
Jules Bianchi was next up for ART, while late improvements from Johnny Cecotto, Esteban Gutierrez and Max Chilton bumped Giedo van der Garde down to 10th.
Traffic was a problem for drivers throughout the 30-minute session, as they all constantly backed up looking for clear track. Barcelona race winner Charles Pic was involved in the most serious moment, when Luiz Razia drove into the back of the Frenchman, who was going slowly in the swimming pool section of the lap.
The other race winner from Spain, Fabio Leimer, had several hot laps ruined. He set the fastest time through the first sector on multiple occasions, but never got a clear lap on his way to 12th place. The Swiss then spun at Mirabeau four minutes from the end.