Francesco Bagnaia went fastest in Friday practice for the potentially decisive Malaysian MotoGP, with title rival Jorge Martin pipped into second as he crashed out.
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Spain's Martin is 17 points ahead of Italian defending champion Bagnaia in the title fight, with just one race weekend left after Malaysia.
Pramac Ducati's Martin can mathematically clinch his first world crown this weekend, with a sprint race on Saturday and a grand prix on Sunday.
But in hot and humid conditions at Sepang, the day belonged to Bagnaia as he fired a warning shot ahead of a critical weekend.
The Ducati ace was easily fastest in morning practice and trumped his rival in the afternoon too, topping the timesheets with a blistering 1min 57.679sec.
That was just 0.050sec ahead of Martin, who went down with just under three minutes left in the session, skidding off his bike to end his afternoon a little prematurely. He was unhurt.
The title tussle is now a straight shootout between Martin and Bagnaia.
Third-fastest was Bagnaia's teammate Enea Bastianini, with Maverick Vinales fourth and Alex Marquez fifth.
Six-time MotoGP world champion Marc Marquez was 10th.
Andrea Iannone, on his return to MotoGP after a four-year doping ban, was 21st for the Valentino Rossi-owned Ducati-VR46 team.
Bagnaia won the grand prix at a wet Buriram in Thailand last week, ahead of Martin, to shave his rival's lead and keep the championship alive.
The season finale is scheduled for Valencia in a fortnight, but top riders, including Martin and Bagnaia, have called for the event to be moved because of deadly flooding in that part of Spain.