Nico Rosberg maintained Mercedes' front-running form, in timed sessions at least, as he posted the fastest time in morning practice on the streets of Monaco.

The German, who claimed the last two pole positions in Bahrain and Barcelona to add to Lewis Hamilton's top spot in China, posted a 1min 16.195secs effort - marginally faster than 2012 - to lead Ferrari's Fernando Alonso by less than a tenth of a second, while Romain Grosjean swapped places with Felipe Massa to claim third spot in the final moments.

Rosberg hardly appeared to be trying as he set the pace, efficiently going about his business at the wheel of the W04. Others, meanwhile, found the going a little tougher, including Mercedes team-mate Hamilton, who was among those to take to the run-off areas during the 90-minute session.

In truth, the session was only 60 minutes as, barring installation laps, there was little action to speak of in the opening half-hour. Once the cars took to the track in anger, however, the remaining time seemed to fly by as the Mediterranean produced perfect conditions for watching motorsport.

While Rosberg set the pace, Ferrari looked equally comfortable, with Alonso contending for top spot until running into traffic on his fastest lap. The Spaniard's 1min 16.282secs effort was still a tenth clear of Grosjean's late improvement, while Massa and fifth-placed Hamilton were within three tenths of the pace.

The surprise of the session, meanwhile, was Pastor Maldonado. While the Venezuelan is acknowledged as something of a Monaco master, this year's Williams has not been helpful to his cause. Still without a point after five rounds, the Grove team took the decision to reintroduce its 2012 nose section, complete with ugly step, on Maldonado's car for the opening session, while rookie team-mate Valtteri Bottas retained the 2013 design. The Finn, on his first Monaco appearance in any formula, was 17th fastest.

Behind Maldonado, Red Bull and McLaren combined to fill out the top ten, with Mark Webber edging Jenson Button, Sergio Perez and team-mate Sebastian Vettel.

Kimi Raikkonen was a couple of tenths behind the world champion, and looked more ragged than Lotus team-mate Grosjean, despite the Frenchman being the one to find the run-off at the chicane. Similarly, Paul di Resta got the better of Force India colleague Adrian Sutil, although the German's cause wasn't helped by a spin in which his brushed his rear wing endplates against the Armco.

Half a second separated Sutil from fellow countryman - and Force India predecessor - Nico Hulkenberg, while Jean-Eric Vergne slotted in 15th, ahead of the second Sauber of Esteban Gutierrez, Bottas and Toro Rosso team-mate Daniel Ricciardo, the Australian having also clipped the barriers early on.

At the back, Giedo van der Garde got the better of the battle between Caterham and Marussia, leading team-mate Charles Pic by a couple of tenths, while Jules Bianchi and Max Chilton - the Briton having narrowly avoided a collision with the pitting Felipe Massa - found themselves adrift at the back.