Red Bull's Daniel Ricciardo headed the times in first practice at the Singapore Grand Prix as championship leader Lewis Hamilton was fourth fastest.
Ricciardo led Ferrari's Sebastian Vettel by 0.109 seconds, with Max Verstappen third quickest in the other Red Bull.
Hamilton was 0.415secs off the pace in his Mercedes but set his time earlier in the session when the track was not as fast.
His team-mate Valtteri Bottas was sixth, behind Force India's Sergio Perez, complaining of a lack of rear grip.
Fernando Alonso signalled what is expected to be one of McLaren's better weekends on this low-grip and slow-speed track where a lack of engine power is not as big a handicap at some others by setting eighth fastest time, behind Ferrari's Kimi Raikkonen.
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The session will be followed by a series of announcements about engine and driver movements in F1 - with McLaren's divorce from Honda first to be confirmed, followed by Toro Rosso's new deal with Honda for 2018, McLaren's switch to Renault and the French team's signing of Carlos Sainz.
Mercedes are expected to struggle this weekend and Ferrari are the favourites on track that seems perfectly suited to their car.
Hamilton and Bottas led the way early on but the Red Bulls were quick as soon as they hit the track and the Mercedes drivers were not able to match their times.
Ricciardo and Verstappen were one and two for a while - 0.121secs apart - before Vettel split them with about 20 minutes to go.
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But the first session is not fully representative of the cars' potential as it is held in daylight, while qualifying and the race are at night.
So the second session, which starts at 13:30 BST, or 20:30 local time, will give a clearer picture of where the teams stand.
Behind Alonso, Renault's Nico Hulkenberg and Toro Rosso's Daniil Kvyat completed the top 10, ahead of McLaren's Stoffel Vandoorne and the second Renault of Jolyon Palmer.
Sainz sat out the session, replaced by Indonesian Formula 2 driver Sean Gelael, who was 18th fastest.