Nico Rosberg laid down an early marker on Thursday when he topped the times for Mercedes in opening free practice for this weekend's 71st Monaco Grand Prix.
The 27-year-old German, seeking a hat-trick of pole positions, narrowly led a closely-contested session that suggested both Ferrari and Lotus will be contenders for the front row on Saturday.
Rosberg clocked a best lap in one minute and 16.195 seconds to confirm his bid for a third straight pole after claiming the prime spot in Bahrain and Spain.
His best lap was fast enough to edge out two-time world champion Spaniard Fernando Alonso of Ferrari by just 0.087 seconds.
Most of the fastest times were delivered midway through the 90-minute session run in mild, dry conditions under a blue sky - a welcome change after Wednesday's rain storms.
There was little action in the opening 30 minutes before the leaders emerged. Brazilian Felipe Massa swapped top spot with Rosberg a few times before Alonso took over and was then usurped briefly by Frenchman Romain Grosjean for Lotus.
Grosjean was one of the drivers to brush the barriers and require minor running repairs as they adjusted their precision settings to the unforgiving demands of the unique and historic Mediterranean street circuit.
Grosjean wound up third fastest ahead of fourth-placed Massa with Rosberg's Mercedes team-mate Briton Lewis Hamilton completing the top five.
Venezuelan Pastor Maldonado, a self-styled Monaco specialist, was sixth for Williams ahead of Australian Mark Webber in the leading Red Bull, Briton Jenson Button and his McLaren team-mate Mexican Sergio Perez, and defending triple world champion German Sebastian Vettel in the second Red Bull.
The only significant incident came when German Adrian Sutil spun and gave his Force India a heavy knock at Sainte Devote in the closing minutes. Hamilton had earlier skated down the escape road at the same corner.