Red Bull's Daniil Kvyat stole the limelight and upstaged record-seeking Lewis Hamilton when he timed quickest in smog-hit free practice for the Singapore Grand Prix on Friday.
The 21-year-old Russian was the surprise pace-setter with a leading time of 1min 46.142 sec at the Marina Bay Street Circuit, ahead of Ferrari's Kimi Raikkonen in second.
Daniel Ricciardo was third quickest in the second Red Bull with Hamilton, bidding to equal a clutch of milestones set by the late Ayrton Senna, down in fourth on the timesheets.
The unusual make-up of the leaderboard, in a season dominated by the Mercedes cars of Hamilton and Nico Rosberg, hints at an unpredictable weekend in prospect.
"We're not happy with the performance today," fumed Mercedes boss Toto Wolff. "We haven't really made the tyre in the way we should on the one-lap performance or the long run either.
"We've seen some spectacular performances on the long runs from Red Bull. We just need to get our heads together and assess what's happening."
Championship leader Hamilton is trying equal Senna's record of eight straight pole positions as well as the late Brazilian's career tally of 41 wins from 161 races.
The Briton leads the standings by 53 points from Rosberg, putting him well on course to match his boyhood idol Senna's haul of three world titles.
Haze caused by Southeast Asian forest fires was measured at unhealthy levels during Friday's practice sessions, held under floodlights at the downtown layout.
The smog could cause concern for the weekend, after Jenson Button warned earlier of the potential health risks of strenuous driving in polluted air.
Kvyat took control early in the second session and held firm despite losing his cool and swearing at Romain Grosjean when he found himself blocked by the Lotus man.