Kimi Raikkonen edged out Ferrari team-mate Sebastian Vettel to the quickest time in final Formula 1 practice for the Malaysian Grand Prix.
Raikkonen posted a 1m31.880s on the super-soft compound tyres to finish 0.162 seconds clear of Vettel, with Red Bull's Daniel Ricciardo half a tenth further back in third.
However, there was frustration for Ferrari in the final minutes of the session when Vettel was told to slow and return to the pits.
Championship leader Lewis Hamilton could only manage the fifth quickest time, 0.659s off the pace, with team-mate Valtteri Bottas two tenths quicker in fourth.
Hamilton made a mistake at the final corner on his qualifying-simulation, running deep into the hairpin and failing to improve on his earlier time.
Mercedes brought a substantial upgrade to the W08 to Malaysia, including revised front wing, 'spoon' deflector, bargeboards and floor, but has so far struggled relative to its rivals.
It decided to run split programmes with its drivers in a bid to find answers, with Bottas running the new aero package and Hamilton reverting to the old.
Hamilton had set the early pace, when most ran used super-softs, clocking a 1m32.539s, with Raikkonen and Vettel slotting into second and third respectively.
When the teams bolted on fresh sets of super-softs, which Pirelli says is around 0.8s quicker than the soft, Ricciardo was the first out and went quickest with a 1m32.091s.
Vettel then moved to the top of the pile before Raikkonen usurped him to end the session quickest of all at a dry and muggy Sepang.
Max Verstappen was sixth quickest, 0.040s adrift of Hamilton, with his qualifying-simulation coming late on, but his session was disrupted when he had contact with Jolyon Palmer at the final corner.
Palmer, who had a scrappy session that included an off-track excursion, dived down the inside of the final corner, locking up and hitting the Red Bull.
The impact broke Palmer's front wing with Verstappen reporting a puncture, and both drivers were summoned to the stewards over the incident.
Sergio Perez improved to seventh quickest, 1.329s off the pace, with Felipe Massa eighth only 0.031s adrift in the Williams.
Esteban Ocon and McLaren's Stoffel Vandoorne, who outpaced team-mate Fernando Alonso, completed the top 10.
It was another tough day for Grosjean, whose mechanics worked into the early hours of the morning following his bizarre Friday practice crash.
The Haas driver spun and aborted a flying lap when he picked up a flat-spot as a result of a big lock up into Turn 9 and ended up 18th.
His team-mate Kevin Magnussen had his running limited when the team recalled him to the garage after it suspected a problem with the engine but he got back out on track and clocked the 13th quickest time.
|3||Daniel Ricciardo||Red Bull/Renault||0.211s||16|
|6||Max Verstappen||Red Bull/Renault||0.699s||17|
|7||Sergio Perez||Force India/Mercedes||1.329s||20|
|9||Esteban Ocon||Force India/Mercedes||1.410s||21|
|15||Carlos Sainz Jr.||Toro Rosso/Renault||2.044s||22|
|16||Pierre Gasly||Toro Rosso/Renault||2.326s||24|