Reigning seven time MotoGP world champion Valentino Rossi began 2010 MotoGP testing on top of the timesheets by a comfortable 0.491sec at Sepang on Thursday.

The Fiat Yamaha rider was fastest for much of the day, having recorded his best lap time of 2min 1.411sec on lap 15 of 54.

"Today was very important because this year we have just six days of testing in which to set the bike, so we can not waste any time," said Rossi.

"This first day was very positive, we tried the new bike for 2010, with a different chassis and a new engine, which can do more kilometres, and it is quite fast.

"Yamaha has worked very well over the winter. Now we need to spend time on setting some electronics and other things. This is only the start but it seems we're already in good shape."

Rossi's nearest rival was 2007 world champion Casey Stoner, who also set his best lap time early in the day, the Ducati star's best coming on lap 12 of 42. Stoner will be seeking a cure for chatter tomorrow.

Third fastest was satellite Tech 3 Yamaha rider Colin Edwards, who claimed to be delighted with the latest edition of the YZR-M1, with which he lapped just 0.03sec slower than Stoner and 0.170sec ahead of Suzuki's Loris Capirossi.

Fifth fastest and the final rider within one second of Rossi was the Italian's team-mate and 2009 title runner-up Jorge Lorenzo.

Repsol's Andrea Dovizioso was the top Honda rider in sixth, while Nicky Hayden (Ducati Marlboro), Dani Pedrosa (Repsol Honda) and Mika Kallio (Pramac Ducati) occupied places seventh to ninth.

Hector Barbera caused a surprise by being the fastest of the 2010 rookies in tenth place. The Spaniard steered his Aspar Ducati around the Malaysian Grand Prix circuit in a best time 1.6sec slower than Rossi.

A visibly jet-lagged Ben Spies was twelfth fastest (+1.732sec) and the World Superbike champion claims a good night's sleep, rather than any radical set-up changes, will take him nearer to the front.

Thursday saw intensive heat and humidity at Sepang, with rain finally arriving 20-minutes before the eight hours of scheduled track action had concluded.

Testing continues on Friday.