Gresini Racing's Bastianini avenged his narrow San Marino GP defeat earlier this month by denying Bagnaia a fifth straight win after a photo finish in Alcaniz, fighting off the Ducati rider to win by 0.042 seconds.
Bagnaia cut Frenchman Quartararo's championship lead to 10 points with five races remaining, with Aleix Espargaro also back in the title hunt after completing the podium for Aprilia by passing Red Bull KTM's Brad Binder on the penultimate lap.
Reigning champion Quartararo hit the rear of Marc Marquez's Honda on the exit of turn five before the Spaniard and Takaaki Nakagami made contact, bringing the race to a premature end for all three riders.
"I lost the rear in Turn 3 and Fabio made contact. When I engaged the holeshot device later in the lap, the bike locked and moved to the left because there was a piece of Fabio's bike in the rear," Marquez said.
"It was really unlucky and I want to apologise to both Taka and Fabio."
