MONACO, May 22 (Reuters) - Formula One leader Lewis Hamilton set the pace in first practice for the Monaco Grand Prix on Thursday with Mercedes team mate Nico Rosberg close behind.
The Briton, who will be chasing his fifth win in a row on Sunday, lapped the Mediterranean street circuit with a best time of one minute 18.271 seconds on an overcast morning in the principality.
Rosberg, winner from pole position last year, was a mere 0.032 slower with Australian Daniel Ricciardo the best of the rest for champions Red Bull in third place and 0.235 off Hamilton's pace.
Hamilton leads Rosberg by three points in the standings after four successive one-two finishes. Mercedes have won every race and started them all from pole.
Ferrari's Fernando Alonso, a past winner in Monaco with Renault and McLaren, locked up at Mirabeau at the end of the session but was fourth on the timesheets with Red Bull's quadruple world champion Sebastian Vettel fifth.
The only casualty of the morning was German driver Adrian Sutil, who made contact with the barriers at Mirabeau. Marussia's Max Chilton also stopped on the track.
The session was closely watched for signs of how the new cars, with their heavier V6 turbo hybrid power units, would handle the layout with more torque and less downforce.
Some drivers have warned that Monaco could be the toughest test of the season, with the cars slipping and sliding more around the tight corners on a track where any mistake can be costly.
Rivals Red Bull and Ferrari have also sensed an opportunity, with the performance advantage of the Mercedes engine less of a factor for once.
"Mercedes have been quite strong here over the last couple of years but it's an opportunity," Red Bull principal Christian Horner said before the session started. "We've got a bit of funny weather this weekend and we'll do everything we can." (Reporting by Alan Baldwin, editing by Justin Palmer)