Formula One: Rosberg earns 3rd straight pole, Hamilton makes all-Mercedes front row

MONACO (Reuters) - Germany's Nico Rosberg scorched to his third straight pole position with team-mate Lewis Hamilton completing a front row lockout for Mercedes at the Formula One Monaco Grand Prix on Saturday.

The pole was the fourth in a row for Mercedes, whose qualifying form has faded in the races so far this season, and second successive sweep of the top two places.

Red Bull's three-time champion Sebastian Vettel qualified third, with Australian team mate and last year's winner Mark Webber lining up alongside.

Rosberg could become the first son of a Monaco Grand Prix winner to win the most glamorous race on the calendar - 30 years on from his father Keke's 1983 victory for Williams - but he was not about to get ahead of himself.

"It's always nice to hear statistics like that but it doesn't change much for tomorrow," said the Monaco resident who had dominated every practice session through the treacherous metal-fenced streets.

"In Barcelona we were struggling a lot in the race and I finished 70 seconds behind the winner. We're improving step by step, but there are still a lot of questions remaining. It (the tyre wear) is still our weakness but we'll make the best of it and who knows."

Hamilton, 0.91sec slower than his team-mate, was happy to overcome the problems he experienced in practice and to qualify second for a race that should see just two pit stops rather than the average of four witnessed in Spain.

"I've been struggling all weekend and my pace has been slacking. I've struggled with the car and this is the first time that's happened since 2005," the 2008 winner said, referring to his pre-F1 days.

"It's been a difficult weekend so I'm happy to have been able to lock out the front row for the team. It's been a massive improvement for me."

Ferrari's Felipe Massa did not set a time after crashing in final practice, leaving his mechanics with too much to fix and not enough time to do it. The Brazilian will start last.