Malaysia Grand Prix: Kimi Raikkonen top as Sebastian Vettel breaks down
Lewis Hamilton set only fifth fastest time in final practice as his Mercedes team continued to struggle at the Malaysian Grand Prix.
He was 0.659 seconds off the pace, set by Ferrari's Kimi Raikkonen, with his title rival Sebastian Vettel second.
But Vettel later suffered an engine problem at the end of the session and has a new one for qualifying.
Hamilton was also behind Red Bull's Daniel Ricciardo and Mercedes team-mate Valtteri Bottas, 0.210secs faster.
He had been on for a faster lap, which would have beaten Bottas but ran wide at the final bend.
Mercedes' performance is likely to be a major concern for the team, especially as their two cars were running in different specifications in the session in an attempt to understand the handling problems the drivers experienced on Friday.
An aerodynamic upgrade was taken off Hamilton's car but left on Bottas' - the team saying it was so they could "confirm our confidence in that kit".
The fact that Hamilton ended up slightly slower than Bottas, but should have been slightly faster, would be expected to leave them in a quandary - especially as this is a track on which at the very least they expected to be evenly matched with Ferrari.
One positive for Mercedes is that the gap to the front is much smaller than it was on Friday, when they were 1.4secs off the pace.
Hamilton holds a 28-point advantage over Vettel in the championship after his victory two weeks ago in Singapore, where Vettel crashed out at the start in a pile-up with Raikkonen and Red Bull's Max Verstappen.
Verstappen was sixth fastest on Saturday at Sepang, ahead of the Force India of Sergio Perez, the Williams of Felipe Massa, Perez's team-mate Esteban Ocon and the McLarens of Stoffel Vandoorne and Fernando Alonso.
Verstappen had a contretemps with Renault's Jolyon Palmer at the final corner late in the session, the two colliding with Palmer damaging his front wing and Verstappen being sent into a spin and puncturing his left rear tyre.
The pair were called to the race stewards to discuss the incident, but no further action was taken, the officials deciding both could have acted differently and neither was "wholly or predominantly to blame".