Italy's Valentino Rossi won his fourth successive world championship with a narrow victory in Sunday's Australian MotoGP Grand Prix over main rival Sete Gibernau in Phillip Island, Australia.

"Every world championship has another feeling, another story," Rossi said.

For sure the first one was incredible. The emotion is incredible. But this one for me is the best," added the 25-year-old, who switched from top team Honda to Yamaha this year.

Rossi held a 30-point lead over Spain's Gibernau with two races remaining in the season and needed a top-two placing to clinch the title.

Rossi staged a two-man duel with Gibernau from the start of the 27-lap race and the two men exchanged the lead several times in the final stages.

Rossi's winning margin was 0.097 seconds over the Honda rider with Italy's Loris Capirossi third.

Ducati's Capirossi was more than 10 seconds behind Rossi.

Gibernau sat stony-faced and with his arms crossed beside Rossi during the news conference, unable to conceal his disappointment.

"I tried my best, even though I had this problem with the bike. Valentino did a great race," the Spaniard said.

"Congratulations to him and Yamaha. We are still with a satellite team and a satellite bike. We are doing a great job."

The final race of the season is in Valencia on October 31. Gibernau has 244 points with Rossi on 279.

GREATEST EVER

Rossi's fourth successive Australian GP victory in the premier class has sealed his place among the greatest riders, after adding the 2004 MotoGP world title to his 2003 and 2002 championships and his 500cc title in 2001. The MotoGP class began at the start of 2002.

Rossi has also become the seventh rider to claim four or more premier-class world championships. Italy's Giacomo Agostini won eight, three ahead of Australia's Mick Doohan.

The popular Rossi's world championship victory is the first for Yamaha since American Wayne Rainey claimed his third title in 1992.

It is also the first time a Yamaha rider has scored more than seven victories in a season in the premier class.

Rossi has become only the sixth rider to reach the milestone of 100 podium finishes.

Fans including some from Italy crowded on to the track at Phillip Island, 130km south-east of Melbourne, as Rossi kissed his helmet and threw it into the crowd of more than 40,000.

Spaniard Daniel Pedrosa, 19, became the youngest 250cc world champion on Sunday with a fourth-place finish in the Australian Grand Prix won by Argentina's Sebastian Porto.

Porto won the race in 39 minutes 24.604 seconds on an Aprilia to secure second place in the championship on 256 points with Honda rider Pedrosa on 292.

San Marino's Alex De Angelis finished second on an Aprilia, 5.941 seconds behind Porto, with compatriot Manuel Poggiali in third on another Aprilia.

Italy's 125cc world champion Andrea Dovizioso continued his dominant season with victory in Sunday's Australian Grand Prix by a margin of 0.123 seconds.

The 18-year-old Dovizioso scored his fifth win of the campaign, holding off strong challenges in the closing three laps from Spain's Jorge Lorenzo and Australia's Casey Stoner.