Rossi (left) admits Stoner (right) is a worthy champion

Casey Stoner all but clinched the title by winning in San Marino, where the top three were all on Bridgestones, rather than the Michelin tyres used by Yamaha.

"Yamaha must change something," Rossi told Gazzetto dello Sport. "I prefer to do something else than race if I have to race like this.

"I don't think anyone likes the tyre situation. There's no battle any more."

The Misano track where the race took place is close to Rossi's home town of Tavullia, and his fans turned out in force in the hope of seeing him close the gap on runaway championship leader Stoner.

But Rossi's engine failed on the fifth lap and he was forced to limp back to the pits, as Stoner led from start to finish to open up an 85-point gap in the standings.

Victory in the next race in Portugal on 16 September will see the 21-year-old Australian crowned MotoGP champion in only his second season in the competition.

But Rossi has clearly already given up any hope of winning the title.

"Ducati and Stoner deserve to win this world championship and I take my hat off to them," he said.