Dani Pedrosa dominated the opening MotoGP practice session at Sepang, finishing clear of Repsol Honda team-mate Casey Stoner and championship leader Jorge Lorenzo.
Victory in Japan last weekend saw Pedrosa narrow Lorenzo's lead to 28 points with three races left.
But Lorenzo will win the title if he finishes third at each race, regardless of what Pedrosa does.
Britain's Cal Crutchlow was fifth fastest in Malaysia practice.
Pedrosa posted a time of two minutes 1.621 seconds during the morning session, while the afternoon session was blighted by rain - rendering the times meaningless.
Defending champion Stoner returned from ankle injuries last weekend and looked close to top form in setting the second-fastest time, just over a tenth of a second down on Pedrosa.
Malaysian MotoGP practice times
1. Dani Pedrosa (ESP/Honda) 2min 01.621sec
2. Casey Stoner (AUS/Honda) at 0.152
3. Jorge Lorenzo (ESP/Yamaha) at 0.313
4. Andrea Dovizioso (ITA/Yamaha) at 0.615
5. Cal Crutchlow (GBR/Yamaha) at 0.645
6. Ben Spies (USA/Yamaha) at 0.737
7. Stefan Bradl (GER/Honda) at 0.996
8. Nicky Hayden (USA/Ducati) at 1.133
9. Alvaro Bautista (ESP/Honda) at 1.251
10. Hector Barbera (ESP/Ducati) at 1.600
11. Valentino Rossi (ITA/Ducati) at 1.653
12. Karel Abraham (CZE/Ducati) at 3.003