Sebastian Vettel quickest, Lewis Hamilton eighth in Monaco
Ferrari's Sebastian Vettel set an impressive pace in second practice at the Monaco Grand Prix as Lewis Hamilton ended the session eighth.
Championship leader Vettel was 0.487 seconds quicker than Red Bull's Daniel Ricciardo, with Hamilton more than a second off the pace.
Hamilton was top in the first session but Mercedes were less competitive later, a concern for the weekend.
Jenson Button was 12th for McLaren on his one-off return to Formula 1.
The 2009 world champion, standing in for Fernando Alonso while the Spaniard races at the Indianapolis 500 this weekend, was just 0.035secs slower than and one place behind team-mate Stoffel Vandoorne.
The session was stopped for about 10 minutes midway through after a heavy crash for Williams rookie Lance Stroll.
The 18-year-old Canadian lost the car on the entry to Massenet, one of the fastest and most demanding corners on the track, and smashed into the barriers, badly damaging the right-hand side of the car.
At the front of the field, another close battle looks in store between Mercedes and Ferrari, and Red Bull appear closer than they have been so far this season.
Hamilton was quickest by just under 0.2secs from Vettel in the first session but the silver cars appeared to be struggling for pace in the warmer afternoon conditions.
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Their lack of pace on the qualifying simulation runs prompted them to do an extra performance run on the fastest ultra-soft tyres to try to learn more about the set-up ahead of the crucial qualifying session on Saturday, with Friday being a rest day in Monaco.
Mercedes then did not do the usual race-simulation runs, concentrating instead on pit-stop practice.
However, judging by the brief running the teams did do on race fuel, the Mercedes and Ferrari ran at very similar pace, with Daniel Ricciardo's Red Bull not far behind.
"We didn't have the greatest day," said Hamilton.
"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."
Hamilton's team-mate Valtteri Bottas was 0.025secs slower than the Englishman in 10th place.
"Monaco is always amazing to drive, but this is definitely the best car I've driven around here," Hamilton added.
"Ferrari are very quick again and Red Bull are also looking good this weekend. We're looking forward to a real fight on Saturday."
The Toro Rosso cars of Daniil Kvyat and Carlos Sainz were fourth and fifth ahead of the second Red Bull of Max Verstappen, who had an action-filled day.
The Dutchman bounced over the kerbs at the first Swimming Pool chicane in the first session, damaging the car's floor, and in a lurid start to the second session he cut the second Swimming Pool corner, where he crashed last year in qualifying, to avoid another accident.