Sepang - 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 1 minute 32.539 seconds 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 but suffered engine problems late on.
Red Bull's Daniel Ricciardo was third with Valtteri Bottas, who retained Mercedes' new bodywork as the team split their strategy, improving to fourth after limping home 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 will compete in qualifying later with a new engine after slowing down and returning to his garage at the end of the session stuck in second gear.
Ferrari said it was an electrical problem and opted to fit a new, upgraded engine for qualifying.
The problem now for Ferrari is that it is Vettel's final permitted power unit with a quarter of the season's 20 races still remaining after Malaysia.
It puts the German four-time world champion on the brink of penalties if he were to have another engine failure.
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.
To make matters worse, a final bend collision with Renault's Jolyon Palmer saw both drivers called in front of the stewards to explain the incident.
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.
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.
Leading lap times from the third free practice session on Saturday for the Malaysian Grand Prix:
Kimi Raikkonen (FIN) Ferrari 1:31.880, 2. Sebastian Vettel (GER)
Ferrari 1:32.042, 3. Daniel Ricciardo (AUS) Red Bull 1:32.091, 4.
Valtteri Bottas (FIN) Mercedes 1:32.329, 5. Lewis Hamilton (GBR)
Mercedes 1:32.539, 6. Max Verstappen (NED) Red Bull 1:32.579, 7. Sergio
Perez (MEX) Force India 1:33.209, 8. Felipe Massa (BRA) Williams
1:33.240, 9. Esteban Ocon (FRA) Force India 1:33.290, 10. Stoffel
Vandoorne (BEL) McLaren 1:33.321, 11. Fernando Alonso (ESP) McLaren
1:33.530, 12. Lance Stroll (CAN) Williams 1:33.538, 13. Kevin Magnussen
(DEN) Haas 1:33.787, 14. Nico Hulkenberg (GER) Renault 1:33.871, 15.
Carlos Sainz Jr (ESP) Toro Rosso 1:33.924, 16. Pierre Gasly (FRA) Toro
Rosso 1:34.206, 17. Jolyon Palmer (GBR) Renault 1:34.475, 18. Romain
Grosjean (FRA) Haas 1:34.914, 19. Marcus Ericsson (SWE) Sauber 1:34.936,
20. Pascal Wehrlein (GER) Sauber 1:35.045