Teammate Francesco Bagnaia, who lies second in the championship standings, was just 0.018 seconds behind as Ducati grabbed the limelight under heavy clouds but no rain in Buriram.
The dry conditions are expected to change to wet this weekend, giving teams and riders a headache over tyre selection and setup.
Jorge Martin was third in Friday’s practice, while eight-time world champion Marc Marquez delighted his fans by sealing his comeback from surgery with the fourth-fastest time.
Championship leader Fabio Quartararo (Yamaha) was fifth as he struggled to hold off the challenge of Bagnaia, just 18 points behind him in the standings.
In Moto2 practice, Thai rider Somkiat Chantra was fourth fastest with a time of 1:36.597s. Somkiat managed to record a competitive time despite suffering a fall during his second practice lap. Fellow Thai Moto2 rider Keminth Kubo clocked 1:37.465s to land 22nd.
Qualifying for grid positions will be held on Saturday before race day at Chang International Circuit on Sunday.