Bastianini came in the shadow of Francesco Bagnaia after a nerve-racking duel in the closing laps to deny the works Ducati rider a fifth straight win by four tenths of a second.
With Quartararo the victim of the chaos on the first lap, Bagnaia came under 10 points behind Yamaha’s defending champion with five races to go.
Aprilia’s Aleix Espargaro finished third at the end of a breathless 15th round of the championship in which six-time world champion Marc Marquez’s hopes of a dream comeback after 112 days turned into a nightmare.
It was Marquez’s Honda that snapped Quartararo, causing the hapless Frenchman to slide down the track.
Seconds later, Honda’s Japanese rider Takaaki Nakagami came on deck after reconnecting with Marquez, who had to stop with damage to his bike.
Marquez was back for the first time since Italian MotoGP after further surgery on the right arm he injured in 2020 and had to stop damaging his bike.