Giacomo Agostini, the most successful grand prix motorcycle racer of all time, doubts Valentino Rossi's chances of claiming a tenth title.

'Ago' won 15 championships in all classes during the 1960s and '70s, including a record eight in the premier 500cc class.

Rossi, who already holds the record for most 500cc/MotoGP wins, is just one title short of Agostini in the premier-class, and also claimed previous honours in the 125 and 250cc series.

After two winless seasons at Ducati, Rossi is returning to Yamaha - with whom he enjoyed the majority of his MotoGP success.

Agostini believes Rossi can still be a formidable opponent, but isn't sure he can overcome the likes of team-mate and reigning double champion Jorge Lorenzo. quotes Agostini as saying: "I'm convinced that Valentino Rossi isn't finished as a racer, but I think it will be difficult for him to win the next championship."

"It has been two years since Valentino won a race, and three since he last won a world title, in 2009. Beyond that, he is also two years older.

"Age does matter - otherwise I would still be racing!"

Angel Nieto, a 13-time title winner in the 125 and 50cc classes, is the only rider between Agostini and Rossi on the list of all time championship wins.

Rossi will be 34 by the start of the 2013 MotoGP season.