Max Verstappen and Daniel Ricciardo achieved a Red Bull one-two in the first free practice session on Friday as Mercedes again struggled to match the pace of their nearest rivals at the Singapore Grand Prix.
Dutchman Verstappen, 18, lapped the Marina Bay Street Circuit in one minute 45.823 seconds to edge out his Australian team mate by 0.049 seconds. Ferrari's Sebastian Vettel was next fastest, 0.464 off the pace.
As was the case a year ago when Vettel stormed to victory, the superior power of the Mercedes appears to have been negated by the high downforce and slow nature of the Singapore track as Lewis Hamilton and Nico Rosberg could only finish fourth and fifth respectively.
Championship leader Hamilton was 0.603 seconds behind Verstappen. Practice for Rosberg ended prematurely when he damaged his front wing after crashing late in the 90-minute session.
The Mercedes duo set their fastest times on the more durable soft compound tyres, however, with the top three all going quicker on ultrasofts.
The session started in the twilight as the cars went out for their first stints at the end of a sweltering day but McLaren's Jenson Button ground to a halt towards the end of his installation lap, bringing out the virtual safety car.
"Yes, it's stopped," the Briton lamented over the team radio. "I'm not going to make it back to the pits."
All the other cars returned to the pits as the Briton was pushed a few hundred metres to his garage by marshals and crew members.
Mercedes were first out after Button pitted and Hamilton's car was fitted with a 'halo' for the first time as Formula One continues to experiment with the driver cockpit safety device.
Verstappen, already earning a reputation as an aggressive driver, brushed the wall early but when he avoided all the street furniture on his next lap, he leapfrogged Hamilton at the top of the timesheets.
After a mid-session lull when all the cars returned to the pits to hand back their first set of tyres, Rosberg soon had the track to himself and immediately took almost a second off Verstappen's early time.
The Red Bulls then went out on new sets of ultrasofts to dominate the remainder of the session, with Vettel also going quicker than Mercedes on his final stint.
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 "Formula One, Singapore GP, Red Bull, Max Verstappen, Nico Rosberg, Daniel Ricciardo,")
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