Max Verstappen and Sebastian Vettel set the fastest times in the opening practice session for the Singapore Grand Prix on Friday, the duo lapping marginally quicker than championship leader Lewis Hamilton in what could prove to be a precursor for a three-way battle for supremacy on Sunday.
Red Bull's Vestappen crossed the line in 1:40.259, just 0.167s ahead of Ferrari's Vettel in a session that was held up by a late crash involving Hamilton's Mercedes team mate Valtteri Bottas which caused the action to be red flagged for 10 minutes.
Under overcast yet clear skies as the threat of haze subsided due to changing winds, the late afternoon conditions will be markedly different from those the drivers will experience under the floodlights when they set about qualifying for Sunday's night race.
Championship leader Hamilton has won in Singapore four times and is favourite to prevail once more as he marches towards a sixth world title, but the Briton spent most of the session completing race simulations on the durable hard tyre, yet he still managed to clock a best time just 0.666 seconds behind Verstappen.
Both the front runners were using the fastest soft compound for their best laps.
Bottas, who was fourth fastest overall, may have been even closer to the Dutchman had he not crashed into the wall as he overshot Turn 19 on a flying lap and clattered into the barriers.
The Finn, who was also racing on hard tyres, was unhurt in the incident and apologised to his team over the radio.
"I just lost the rear, maybe some overheating," he said. "My bad."
Vettel's Ferrari team mate Charles Leclerc, who has won the last two grand prix, also had a session to forget, the 21-year-old Monegasque suffering a gearbox issue that forced him to limp to the pits halfway through the session as his times suggested he was as quick as the front runners.
Leclerc was 19th fastest of the 20 drivers when his afternoon came to a premature end and his team will be hoping to get his car up and running for the second practice session starting at 2:30pm (CAT) on Friday.
Hamilton, who leads Bottas by 63 points with seven rounds remaining, is likely to aim for setting faster times in the second session, when conditions are closer to those expected during qualifying on Saturday night.