Marquez outpaced the four Yamaha M1s by firing in the only legal sub-1m31s lap of the session, ending up 0.228s clear of nearest rival Fabio Quartararo.
Yamaha works rider Maverick Vinales had taken over out front from the get-go and reinforced his lead with each of his first four flying laps, ending up on a 1m31.248s by the time he pulled into the pits.
But his fellow Yamaha rider Quartararo stayed out and soon improved to a 1m31.150s, with Marquez – who had begun the session by practicing bike swap ahead of a possible flag-to-flag race – also jumping ahead of Vinales.
On his next attempt, Marquez cleared Quartararo too, stringing together a 1m30.921s that would keep him top at the chequered flag – with his French rival only able to improve his previous best time by 0.001s in the aftermath.
Vinales did lap faster the Marquez after rejoining the circuit, but his 1m30.801s effort was chalked off for track limits abuse, and he thus settled for third in the classification.
His fellow works rider Valentino Rossi and Quartararo’s teammate Franco Morbidelli made it four Yamahas in the top five, finishing four and five tenths adrift of Marquez respectively.
Andrea Dovizioso was the lead Ducati in sixth, despite being forced to end his warm-up early when he pulled into the pits with smoke coming from the front area of his GP19 with several minutes to go.
Joan Mir led Suzuki’s efforts in seventh, two spots up on teammate Alex Rins, with the pair split by Aprilia’s Aleix Espargaro.
Jack Miller made up the top 10 on his Pramac-run GP19, outpacing the other works bike of Danilo Petrucci, who had to settle for 13th – one spot behind the lead KTM of Miguel Oliveira.
Despite Marquez setting the pace, the rest of the Hondas continued to struggle. Takaaki Nakagami was 11th on the year-old RC213V, with his LCR teammate Cal
Crutchlow eight places lower on the works-spec bike, while Marquez’s factory teammate Jorge Lorenzo made up the order in 22nd, 2.087s off Marquez’s pace.