World championship leader Lewis Hamilton reverted to an old aerodynamic package on Saturday but again struggled to keep pace with the Ferraris in the final practice session for the Malaysian Grand Prix.
After a torrid Friday with a new set-up which Mercedes boss Toto Wolff dubbed "one of the worst he could remember" Hamilton clocked 1min 32.539sec for fifth place.
It was almost seven-tenths of a second behind Kimi Raikkonen, who topped the timesheets, and half a second behind title rival Sebastian Vettel, who was second-fastest.
Red Bull's Daniel Ricciardo was third with Valtteri Bottas, who retained Mercedes' new bodywork, improving to fourth after limping in seventh in both of Friday's sessions.
Vettel, who lies 28 points behind Hamilton in the title race, smashed the all-time Sepang lap record Friday just ahead of Raikkonen.
And the Ferrari pair again dominated, although Vettel gave his team a scare ahead of qualifying later Saturday by slowing down and returning to his garage before the end of the session.
Hamilton's car struggled for downforce on Friday and he was fighting for balance again despite going back to the old set-up that had won him the Singapore Grand Prix a fortnight ago.
He did go faster than Max Verstappen who was left with little to celebrate on his 20th birthday after some ragged laps left him only in sixth place on the timesheets.
Earlier Formula One inspectors gave the Sepang circuit the all-clear after a loose drain caused a shocking tyre explosion during practice on Friday.
Romain Grosjean completed the practice session unscathed but was only 18th quickest in his rebuilt Haas which had been involved in the freak incident on Friday.
A drain grille, which should have been welded down securely, popped up at a kerb on turn nine after Raikkonen drove over it.
Grosjean hit it at high speed, shredding his rear tyre and flinging him off the track into the barriers.
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