Charles Leclerc set the pace ahead of Ferrari team-mate Carlos Sainz in a tightly contested opening practice session at the Singapore Grand Prix.
Leclerc posted a best time of 1:33.350 on the soft tyre to top the timesheet, but Sainz was close behind and 0.078s off his team-mate.
Max Verstappen, bidding for a record-extending 11th win in a row this weekend, was third in the lead Red Bull, 0.126s off Leclerc's benchmark.
Driving a heavily-updated McLaren, Lando Norris was fourth fastest and just ahead of Lewis Hamilton's Mercedes.
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With Practice One taking place in daylight, the session is hard to judge representative times ahead of the key night sessions of Saturday qualifying and Sunday's race, but with less than two tenths of a second covering the top five drivers and four different teams it gives hope of a closely-fought battle for the rest of the weekend.
A clearer picture could emerge when Practice Two gets under way under the floodlights at 2pm.
The session was twice briefly disrupted by yellow flags due to lizards walking onto the track, with Verstappen narrowly avoiding hitting one.
George Russell was sixth in the second Mercedes, a further tenth and a half down on team-mate Hamilton's time.
Russell's hot lap on the soft tyre was disrupted by coming up to the back of a slow-moving Red Bull, with the Briton venting his frustration over team radio.
Sergio Perez was seventh, 0.375s off Leclerc's session-leading time, before a gap of three tenths of a second down to Fernando Alonso, Yuki Tsunoda and Esteban Ocon who rounded out the top 10.
While Norris was fourth in the upgraded McLaren, team-mate Oscar Piastri was down in 19th with the Australian not receiving the new parts until next weekend's Japanese GP.