Nico Rosberg says Red Bull has a genuine advantage over Mercedes at Monaco this weekend and is doubtful his team will be able to make up the difference in qualifying.
Daniel Ricciardo held a 0.6s gap over Lewis Hamilton in Thursday's second practice session and a 0.9s gap over Rosberg. Mercedes often hides its true pace until Saturday's qualifying session, but Rosberg said the times on Thursday suggest Red Bull is the favourite for pole position.
"It's very simple, today they were faster than us," Rosberg said. "We need to dig into it and see if we can find a big chunk of lap time. Of course we can turn up the engine, but we don't have 0.6s of turning up the engine.
"Definitely it's surprising how quick they were. We expected them to be there or there abouts, but not that quick."
Red Bull has closed Mercedes' power advantage this weekend thanks to an upgraded Renault engine in Ricciardo's car and its car is also suited to the tight streets of Monaco. Rosberg said the combination has left him unconvinced Mercedes will take its sixth pole position of the season on Saturday.
"Am I confident [I can take pole]? No, because Red Bull is quicker. I'm really confident we can do a great job to improve the car, but will it be enough? I don't know."