* Rosberg fastest in both sessions
* Hamilton second as Mercedes show their speed (Adds quotes)
By Alan Baldwin
MONACO, May 23 (Reuters) - Nico Rosberg and Lewis Hamilton gave Mercedes a dream start to the Monaco Grand Prix weekend with the fastest times in practice for Formula One's most glamorous race on Thursday.
Rosberg, who has been at home on the twisty streets ever since he was a boy growing up in the principality, set the pace in both sessions with a best time of one minute 14.759 seconds in the afternoon.
That lap compared to last year's fastest time from qualifying set by Mercedes' Michael Schumacher of 1:14.301 and a second faster than last year's best effort in second practice.
Hamilton, the 2008 world champion, was 0.318 off Rosberg's pace in the second session to fuel rising expectations of a fourth successive pole position for Mercedes in Saturday qualifying.
"We completed a lot of laps and I feel comfortable in the car," said Rosberg. "It seems that we are quick again on one lap but we have been working hard again to make improvements to our race pace.
"It will be interesting to see where we are compared to the others over the weekend as it's not really representative today."
Mercedes have suffered from excessive tyre wear on race day, with Hamilton plunging from second to 12th and lapped at the previous race in Spain.
While Rosberg will be chasing his third pole in a row, hat-trick chasing Spaniard Fernando Alonso was second and third fastest respectively for Ferrari.
Alonso, winner of his home race in Spain this month, is hoping to become Ferrari's first Monaco winner since 2001 and the first Formula One driver to triumph in the glamour race with three separate teams.
Team mate Felipe Massa was fourth on a gloriously sunny afternoon with the Mediterranean serving as a sparkling backdrop to the action.
"Even if the feelings are positive and we got through the day without any problems whatsoever, we still don't have a clear idea of the pecking order in the field," said Alonso. "We will have to wait a bit to see where all our rivals really are.
"Until Saturday, no one pushes 100 percent: the barriers constitute too high a threat to take risks in the first free practice sessions."
Frenchman Romain Grosjean was the exception to the rule - crashing into the barriers at Sainte Devote in the afternoon after setting the third fastest lap for Lotus in the morning.
On a historic course threading its way up the hill and into Casino square before dipping and winding down to the darkness of the tunnel and blasting past the moored yachts on the quayside, Venezuelan Pastor Maldonado provided a surprise.
The Williams driver was sixth fastest in the morning, although he faded back in the afternoon, on one of the most treacherous and unforgiving circuits on the calendar.
Williams have not scored a point yet this season, a run of six races in a row including last year's finale, and are in danger of chalking up their worst ever start to a championship this weekend.
Maldonado has form in Monaco, however, winning junior series races there on his way to Formula One and loving the challenge of keeping out of the barriers.
"We feel we have made some progress," said Maldonado, who ran a different setup in the afternoon.
Red Bull's triple world champion Sebastian Vettel, who has a four-point lead in the standings over Lotus's Kimi Raikkonen, was only 10th and ninth quickest after struggling with a KERS problem in the afternoon.
Australian team mate Mark Webber - last year's winner - was seventh and fifth.
"I think traditionally we're never too electric here on Thursdays, so today was actually one of the better ones we've had in the last few years," said Webber.
McLaren's Jenson Button was eighth in both sessions, with Mexican team mate Sergio Perez ninth and 12th, ahead of what could be another difficult weekend for a team struggling to catch up after starting the year well off the pace. (Editing by Ed Osmond and Sonia Oxley)