MONACO (AP) All was going to plan for Mercedes at the Monaco Grand Prix on Thursday morning. A few hours later the head-scratching began.
Lewis Hamilton was fastest in the first practice ahead of Sebastian Vettel, but then finished more than one second behind as Vettel set the fastest time in the second practice. Hamilton finished that in eighth place, with his teammate Valtteri Bottas down in 10th.
An F1 rarity sent ripples through the F1 paddock: meticulous Mercedes got its set-up all wrong when switching to the quicker ultra-soft tires during the afternoon session.
''The difference between the two sessions was night and day. I'm not sure why the tires weren't working,'' a concerned Hamilton said. ''Ferrari are very quick again .... We're looking forward to a real fight.''
Five races into the season, Hamilton trails behind championship leader Vettel by six points with each driver winning two races. Bottas claimed the other.
Since Friday is a rest day - Monaco is the only race to have one - at least the Mercedes engineers and technical staff will have time to figure out the glitch.
''We will have a good think about it,'' technical director James Allison said.
Victory for Ferrari would be particularly sweet in Monaco - where it has not won since 2001.
Vettel, who won his four titles with Red Bull from 2010-13, sounded optimistic that Ferrari can carry the pace into Saturday's third practice and qualifying.
''It's important to have a good feel for the car,'' he said. ''Kimi and I were quite happy with the long run.''
Even the normally inscrutable Raikkonen sounded upbeat after recording the third quickest time in P2 behind Red Bull's Daniel Ricciardo - who got pole position here last year.
''We had two smooth sessions,'' Raikkonen said. ''We did some changes and it always got better.''
By contrast, the mood inside the Mercedes camp was likely to be one of concern. Hamilton was 1.15 seconds adrift of Vettel and Bottas was 1.18 slower.
It's a significant deficit when teams are scrapping for minimal gains.
''We clearly went in the wrong direction,'' Bottas said. ''When the car isn't quite right, you lose a lot of time in Monaco. At least we've learned what not to do with the set-up.''
The Mercedes slump was especially surprising considering Hamilton's smooth first practice.
''I don't know what happened to them today, it was a bit weird,'' Vettel said. ''I'm pretty sure there is a reason and they will be back to full force on Saturday.''
Hamilton has 64 pole positions to his name, one less than F1 great Ayrton Senna, but only one of those in Monaco - where he won last year and in 2008.
Meanwhile, Lance Stroll's miserable run continued. The 18-year-old Canadian crashed into the barriers near Casino Square, bringing the session to a brief halt while his car was lifted off the track.
''I just lost the rear of the car pushing for the limit,'' Stroll said. ''It happens. At least now I know.''
The Williams driver has failed to finish his past three races and has yet to score a point.
In the first session, Hamilton was followed by Vettel, Red Bull's Max Verstappen and Bottas - who is the other driver to win a race this season.
Making his return to F1 as a one-off replacement for McLaren's Fernarndo Alonso - who is skipping Monaco to compete in the Indianapolis 500 - veteran Jenson Button was 12th best in the afternoon and 14th in the morning.
The Williams team carried a (hash)Manchester sticker on the front wing of their cars as a tribute to the 22 people killed and 116 injured in the Manchester bombing on Monday at an Ariana Grande concert.
The Mercedes team said it was also planning to carry a sticker on the front wing.
A minute's silence for the victims is also planned before Sunday's race.