Valentino Rossi claims it will be harder to retain his MotoGP title this year as his rivals will be motivated by his 2008 win.

The Fiat Yamaha rider thinks Ducati Marlboro's Casey Stoner, Repsol Honda's Dani Pedrosa and Rossi's own Fiat Yamaha team-mate Jorge Lorenzo will be his main challengers.

"2008 was difficult - the year I had to give the most of myself in order to win," said Rossi. "In 2009 it will be even more difficult - because my adversaries, who suffered in 2008, are now out for payback and will be looking to beat me. I think it's going to be even harder than 2008."

He added: "I think that the number one rival will be Stoner. Then also Pedrosa will be very fast - and, of course, Lorenzo."

Having tried the prototype Yamaha M1 at the end of last season, Rossi will be looking for progress next time out at Sepang in Malaysia.

He said: "The bike seems better, and I was faster than I was with the 2008 version. I'm very confident, and anyway we're only at the beginning of our work - now we must use these months before April to improve the 2009 M1 even more.

"In Sepang we will use our new bike. I will use my two bikes with the parts which I will use in the race.

"We're working a lot on the engine and we're trying to find a way to improve the acceleration a bit, and I think this is the area we will be concentrating on."