Maverick Vinales led FP1 at the Chang international circuit in Thailand for Yamaha, while Marc Marquez was lucky to escape from a fearsome crash in the closing minutes.
Vinales slowly worked his way up to the fastest time throughout the session having been as low as 15th following the initial runs, deciding to build bit by bit as Marquez took the opposite approach.
The world championship leader-who needs to outscore chief rival Andrea Dovizioso by just two points this weekend to secure his sixth premier class title- immediately set a string of fastest laps early on to lead the field by up to 1.7 seconds at one point.
He eventually settled with a 1:31.187 that looked like it would remain unbeaten, with nearest rival Vinales still roughly half-a-second behind.
Marquez set out to set one final flyer at the end of the session though, but crashed heavily as he entered Turn 7 on his out-lap.
Tech 3 KTM’s Miguel Oliveria also had a nasty incident at the bend less than two minutes after the session began-thankfully uninjured- with Marquez losing the rear of his Honda on entry and suffering a sizeable high-side.
He was helped away by marshals following the crash, and limping away it remains to be seen whether he suffered any weekend-impacting unjury.
This left him defenceless against any improvement from his rivals, with Fabio Quartararo first to take the bait.
The Petronas SRT man squeaked ahead of Marquez to head the times on his final flyer, before Vinales swept in shortly after with a 1:30.979 to secure what would ultimately be the session’s fastest lap.
Quartararo and Marquez therefore followed behind in second and third respectively, while Dovizioso managed the fourth fastest time ahead of fellow GP19 rider Jack Miller on his Pramac-run example.
Valentino Rossi was a strong sixth just clear of Franco Morbidelli on the second SRT Yamaha-making it all four M1’s in the top seven- while Danilo Petrucci secured the eighth quickest lap of the session on his Ducati.
The top ten was rounded out by Joan Mir’s Suzuki and Francesco Bagnaia on the second Pramac Ducati machine.
Alex Rins could only manage the 11th fastest time on the second GSX-RR after suffering a break down into Turn 1, the double ’19 race winner forced to coast back to the pits as a result.
Cal Crutchlow could manage just 18th fastest ahead of Jorge Lorenzo, while KTM look set to struggle with Pol Espargaro the top RC16 man in 13th with team-mate Mika Kallio just 20th.
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