Jorge Lorenzo ended the day on top in Malaysia as MotoGP testing returned ahead of the 2012 season.
Lorenzo - who won the world championship in 2010 - managed a best lap of two minutes 1.657 seconds on his Yamaha, ahead of Dani Pedrosa and British rider Cal Crutchlow.
Current champion Casey Stoner missed the test after injuring his back, but he expects to ride on Wednesday.
Stoner said: "I was stretching and all of a sudden my back just locked up."
The capacity of MotoGP bikes has increased from 800cc to 1000cc this year and Lorenzo - riding for the first time since losing the tip of a finger last year - admitted that he is already enjoying the extra power available.
"It felt better than ever to be back on a MotoGP bike," he said. "The finger is shorter and lighter - that's why I went faster!
"The 1000cc bike is much more powerful so, if you make a mistake. you have enough power to recover the time on the exit. With the 800 you could not make any mistakes."
Nine-time world champion Valentino Rossi endured a poor 2011, but was pleased with his new Ducati after clocking the fifth-best time.
Rossi said: "I am so happy because we fixed a lot of problems that in all of the 2011 season we were not able to improve.
"Braking, corner entry, agility - the bike is a lot better. Ducati have done a great job in a short time."
The first MotoGP of the season will take place in Qatar on 8 April.
Selected MotoGP test times from Sepang:
1. Jorge Lorenzo (Spn) Yamaha 2'01.657
2. Dani Pedrosa (Spn) Repsol Honda 2'02.003
3. Cal Crutchlow (GB) Tech 3 Yamaha 2'02.221
4. Ben Spies (USA) Yamaha 2'02.234
5. Valentino Rossi (Ita) Ducati 2'02.392
6. Andrea Dovizioso (Ita) Tech 3 Yamaha 2'02.751
10. Nicky Hayden (USA) Ducati 2'03.151
15. Colin Edwards (USA) Forward Racing CRT 2'08.240