Andrea Dovizioso topped a red-flagged MotoGP FP2 in Thailand by 0.031 seconds, while Ducati teammate Jorge Lorenzo crashed heavily in the latter stages.
Marc Marquez led the session for most of the 45 minutes, before FP1 pacesetter Maverick Vinales and Dovizioso dethroned him in quick succession in the closing moments.
Pramac's Jack Miller set the early pace in a scorching FP2 with a 1:33.161s, though was immediately usurped by teammate Danilo Petrucci by 0.020s.
Marquez, who was fifth in FP1 after electing not to go for a time attack, shot to the top of the standings soon after on a fresh soft slick with a 1:33.904s, which he improved to a 1:31.188s to go 0.9s clear of the field.
Thailand test pacesetter Dani Pedrosa cut his Honda teammate's advantage down to six tenths with almost 20 minutes of the session gone, with Petrucci returning to third after encountering an early technical issue with his Ducati.
A heavy crash for Ducati's Jorge Lorenzo at Turn 3 with 12 minutes remaining, which appeared to be caused by a mechanical issue, left debris and fluid on circuit, leading to a red flag when Avintia's Xavier Simeon crashed at the same turn moments later.
Lorenzo, who is already nursing a foot injury he suffered at Aragon, was stretchered off circuit and has been taken to the medical centre for checks.
A flurry of fast laps came when the session restarted, with Yamaha's Vinales the first big mover to the top of the timesheets with a 1:31.121s, set using the hard rear tyre.
This stood as the benchmark only briefly, as a soft-shod Dovizioso edged ahead by three hundredths with a 1:31.090s, which remained unbeaten as the chequered flag came out.
Vinales held onto second ahead of LCR Honda's Cal Crutchlow, while Marquez was shuffled back to fourth having once again elected against a late time attack. Petrucci completed the top five.
Andrea Iannone [Suzuki] brushed off a crash at Turn 4 to end the session sixth ahead of fellow faller Alvaro Bautista, who dropped his Nieto Ducati at Turn 3 late on.
Pedrosa was knocked back to eighth in the end ahead of the sister factory Yamaha of Valentino Rossi, who evaluated a new aerodynamic fairing in FP2. Johann Zarco secured the final provisional Q2 spot with a late lap of 1:31.421s on his Tech3 M1 in 10th.
Lorenzo was seventh at the time of his crash, but was pushed back to 16th, while Aprilia's Scott Redding was 21st after a fast early tumble at Turn 4.
