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Bagnaia won a chaotic and ultimately-shortened Moto3 Malaysian Grand Prix on his Aspar-run Mahindra, finishing a comfortable 7.108 seconds clear after early crashes decimated the lead pack.
With his second victory of the season, the Italian reached a target set by Aspar to earn laps on one of the team's MotoGP Ducati's in the Valencia post-season test on November 15.
Moving up to Moto2 next year when Rossi's VR46 outfit steps into the class, Bagnaia is now third in the Moto3 championship with one round remaining.
"I think that was the best race of my life," said Bagnaia.
"It is never easy to ride at the front on your own. You have to learn where the limit is and try to stay as fast as possible.
"I think I have matured because last year I would have struggled to make a race like this, so we have taken a big step forward and I have to thank the team a lot.
"This was the most difficult race of my life but now we have two wins and six podiums from this season.
"Now we can think about Valencia, where thanks to a bet I made with the team for winning two races I'll also get to ride the Ducati."
Jakub Kornfeil finished second and Bo Bendsneyder third at Sepang, the race red-flagged after 13 laps following Maria Herrera's crash at Turn 15.
Earlier, there were pile-ups at Turn 2 and Turn 6 on the opening lap, with champion Brad Binder then among the riders caught out at Turn 7 on lap two.
Binder got back to the pits where his KTM was repaired enough to rejoin the race, finishing three laps down in 17th place.
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