Lewis Hamilton finished down in eighth
Ferrari's championship leader Sebastian Vettel was fastest in second practice at the Monaco F1 Grand Prix.
Mercedes driver Lewis Hamilton finished down in eighth - more than a second behind - having been quickest in the opening session.
"We didn't have the greatest day," Hamilton told the BBC.
"Practice one was actually really good and we were quickest, but then in second practice we just struggled to extract the grip from the tyres and we were sliding around a lot.
"I'm not sure why the tyres weren't working but the difference between the two sessions was night and day. We've got some work to do ahead of Saturday that's for sure but I'm confident the team can work it out."
Britain's former world champion Jenson Button was 12th.
He's making a one-off return for McLaren while Fernando Alonso competes in the Indianapolis 500.
Sebastian Vettel after today's practice session in Monaco. Image: HOCH ZWEI/DPA/PA Images