Mercedes F1 team knows Red Bull Monaco Grand Prix threat is real

Daniel Ricciardo finished fastest in second practice, 0.606s quicker than Hamilton with Rosberg three tenths further back.

Mercedes traditionally turns its engines up for qualifying but Rosberg is concerned that may not be enough make up that deficit.

"It's very simple, today they were faster than us," he said.

"We need to dig into it and see if we can find a big chunk of lap time.

"We can still turn up our engine but we don't have six tenths of turning up the engine.

"It is definitely surprising how quick they were.

"We expected them to be there or thereabouts but not that quick. We have some homework to do."

When asked if he saw Red Bull as a genuine threat, Rosberg said: "Yes definitely.

"If today was qualifying they would be in front of us so we definitely need to find some more pace - and it's not going to be easy.

"Red Bull is quicker but I'm really confident we can do a great job and improve the car - if it is enough, I don't know."

Hamilton said he did not feel he had the smoothest practice session but echoed his team-mate's view of Red Bull's form.

"I don't feel like I got a great lap so there's still time in me," he said.

"Not six tenths, but still time. The gap is definitely closer than it looks. You'll see us within tenths of a second.

"They are very much a threat. They're very, very fast.

"I'm not surprised because the power doesn't make a difference here and I've always known they have a good car.

"Did we expect them to be as fast as they were today? Perhaps not. Maybe a bit closer."