Nico Rosberg has admitted to be taken aback by the pace shown by Daniel Ricciardo's Red Bull on the opening day of the Monaco Grand Prix - and is concerned that Mercedes won't be able to close the gap this weekend.

After a familiar Mercedes 1-2 in opening practice on Thursday morning, Ricciardo - armed with the latest-spec Renault V6 - set a blistering pace during the afternoon session, taking the target time down to 1m 14. 607s while the best Mercedes could manage was Lewis Hamilton's 1m 15.213s effort.

While Rosberg admitted that Mercedes could still call on some extra power, as it usually does on a Saturday, he doubted whether it would be enough to bridge the gap shown on Thursday's timesheets.

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"It's very simple - today, they were faster than us!" the world championship leader conceded, "Definitely, it's surprising how quick they were - we expected them to be there or thereabouts, but not that quick! We need to dig into it and see if we can find a lot of lap-time. Of course, we can still turn up the engine, but we don't have six-tenths of turning up the engine..."

Asked whether he thought Red Bull's pace was for real, Rosberg said he had no doubts, and admitted that his quest for a fourth successive Monaco win might be under threat from more than just his own team-mate.

"For sure [it's genuine pace]," he confirmed, "They're maybe not six-tenths faster but, at the moment, it looks like Red Bull is the fastest car out there so we have a lot of homework to do to find some more performance. [RBR] is definitely a threat [for the whole weekend] - if today was qualifying, they'd be in front of us! We definitely need to find some more pace and that's not easy."

While equalling Ayrton Senna's record of four Monaco wins in a row may not register highly amongst Rosberg's ambitions, he makes no secret of winning on his home patch, but definitely showed signs of doubt heading into the race's traditional day off.

"Confident after today? No, because Red Bull is quicker," he confided, "But I am really confident that we can do a great job and improve the car. Will it be enough though? I don't know..."