Red Bull driver Daniel Ricciardo pipped Ferrari's Sebastian Vettel to top spot in the opening Formula 1 free practice session for the Singapore Grand Prix.
Ricciardo posted a 1m42.489s on the second flying lap on his second set of ultra-soft tyres to secure the fastest time with 26 minutes remaining of the one-and-a-half hour session.
Vettel, also on his second flier on ultra-softs, posted a time 0.109s slower to take second five minutes later to deny Red Bull a one-two by relegating Max Verstappen to third place.
Mercedes drivers Valtteri Bottas and Lewis Hamilton set the pace during the early stages of the session using super-soft Pirelli rubber, when there were a few damp patches on the circuit, before they were shuffled back by Verstappen and Ricciardo's first runs.
Bottas then capitalised on a near-empty track on fresh ultra-softs to climb to second briefly, but he was eventually pushed down to sixth, half-a-second slower than Hamilton and behind Force India driver Sergio Perez, who set his time with 15 minutes to go.
During the long runs, Bottas complained of a lack of rear grip, while Hamilton had a brief off at the slow Turn 14 right-hander.
Kimi Raikkonen ended up seventh, lapping 1.245s slower than pacesetter Ricciardo.
Fernando Alonso was eighth for McLaren ahead of Renault's Nico Hulkenberg, with Toro Rosso driver Daniil Kvyat rounding out the top 10 after Stoffel Vandoorne failed to improve by enough with his late-session pace to get ahead of the Russian.
Felipe Massa was the best-placed of the Williams drivers in 14th, while Lance Stroll completed only 10 laps before pitting with a gearbox problem and ended up 17th.
Massa was ahead of the two Haas drivers, who were the only ones not to use ultra-soft Pirellis and concentrated on super-softs.
Romain Grosjean set his best time with five minutes remaining, although suffered three off-track moments during the session - taking to the run-off at Turn 1 early on before a spin at Turn 16 shortly after and then going off at Turn 18 late on.
He was just over three-tenths faster than Antonio Giovinazzi, the latter sitting in for race driver Kevin Magnussen during FP1.
Formula 2 driver Sean Gelael made his grand prix weekend debut in place of Carlos Sainz Jr at Toro Rosso for the first free practice session.
The 20-year-old Indonesian, who is currently 15th in the F2 standings, was 18th fastest with a 1m45.570s - complaining of a possible problem with the front left that caused "no grip" in the final 20 minutes.
He outpaced the two Saubers, with Marcus Ericsson - who suffered a spin in the Turn 18 left-hander early in the session - 0.187s ahead of team-mate Pascal Wehrlein.
|1||Daniel Ricciardo||Red Bull/Renault||1m42.489s||-||21|
|3||Max Verstappen||Red Bull/Renault||1m42.610s||0.121s||23|
|5||Sergio Perez||Force India/Mercedes||1m43.423s||0.934s||25|
|10||Daniil Kvyat||Toro Rosso/Renault||1m44.220s||1.731s||25|
|13||Esteban Ocon||Force India/Mercedes||1m45.053s||2.564s||25|
|18||Sean Gelael||Toro Rosso/Renault||1m47.570s||5.081s||26|