The top two may have been the same, but at least there was a whiff of a suggestion Lewis Hamilton and Nico Rosberg may not have it all their own way at this weekend's Monaco Grand Prix.

Championship leader Hamilton, aimng to become only the seventh driver in Formula One history to win five consecutive grands prix, finished just a whisker ahead of Rosberg come the conclusion of the opening 90-minute practice session around the streets of the principality.

For once, though, the Mercedes duo did not blow away their rivals as they have achieved in most other practice outings in what has so far been a dominant campaign.

Five successive poles between the pair, five straight wins and four consecutive one-twos have been the story of one of the most crushing starts to this or any other season.

After Rosberg produced a clean sweep last year in topping the times at the end of all three practice sessions, qualifying and then taking the chequered flag, Hamilton managed to get his nose in front on this occasion by just 0.032secs.

In posting a time of one minute 18.271secs, Hamilton finished just over two seconds off Rosberg's best in FP1 a year ago to underline the gap between last year's V8 power and this season's 1.6-litre V6 turbo-chargers is slowly being reduced.

Behind them was Red Bull's Daniel Ricciardo, with the Australian just 0.235secs adrift, offering a glimmer of hope of ending Mercedes' run of results.

With the tight, twisty nature of the track more appropriate for high downforce than outright power, with Mercedes having a stranglehold on the latter, the former may yet come to Red Bull's aid.

There was a gap then, however, with Fernando Alonso in his Ferrari 0.659secs off the pace, followed by the second Red Bull of reigning four-times champion Sebastian Vettel, with the German 0.772secs down.

No other driver came within a second of Hamilton, with Ferrari's Kimi Raikkonen the best of the rest in sixth and 1.196secs behind.

McLaren's Jenson Button was down in 11th position, 1.762secs adrift, finishing two places and a quarter of a second behind team-mate Kevin Magnussen.

For his first outing around Monaco, Toro Rosso's Russian rookie Daniil Kvyat was 18th and 2.643secs down.

Marussia's Max Chilton, currently on a record run of 24 consecutive finishes since his F1 debut, managed just seven laps due to an early technical failure, leaving him at the bottom of the standings, 7.5secs back.