Nico Rosberg says Mercedes is wary of the threat posted by a "very strong" Red Bull squad after Daniel Ricciardo led the way during the second practice session in Monaco.
Rosberg was third quickest in the second session as Ricciardo finished 0.606s quicker than Lewis Hamilton at the head of the pack.
Rosberg, who is aiming for his fourth Monaco win in succession, says he is keeping an eye on rivals Red Bull.
"The sessions were a bit messy, with quite a few Virtual Safety Car periods - but I was able to learn a lot on the longer runs we managed to do which was a positivem" he sauid.
"As expected, Red Bull look very strong here. We have some more homework to do during tonight and tomorrow to find some time - but I can't wait for Saturday and the qualifying battle."
Hamilton, chasing his first Monaco win since 2008, was encouraged by Mercedes' pace across the sessions.
"It's been a relatively good day," he said. "Although the new tyres are definitely softer than the other compounds, they still feel pretty hard around here.
"It's not as easy to get them in the window as we would hope. But generally it's been a really positive start.
"We've got everything we needed done in both sessions and our pace looks good, so hopefully that continues through the rest of the weekend."