Marquez 1-2 as Marc seizes pole for MotoGP season opener
The Spanish great, who has joined double world champion Francesco Bagnaia at the Ducati factory team, will also be on pole for the 13-lap sprint race later Saturday.
In sizzling temperatures, Marquez clocked a leading time of one minute, 28.782 seconds in qualifying in a statement of intent on his Ducati debut weekend.
Alex Marquez, also on a Ducati, took second at 0.146secs off his older brother's time.
Italy's Bagnaia, who struggled badly in Friday practice, will start from third on the grid.
Fourth was Australian Jack Miller on his Yamaha and fifth was MotoGP debutant Ai Ogura in an impressive performance for the Japanese rookie.
The 32-year-old Marquez went quickest in opening practice on Friday before being pipped by brother Alex in the afternoon session.
The first points of the campaign are up for grabs in the sprint race, before the main event at Buriram on Sunday.
Marquez and Bagnaia have been hailed as a dream team but some observers have cautioned that the high-octane mix of two fast high-profile racers could prove too combustible over the 22-race season.
Marquez enjoyed a strong pre-season on his new bike, having joined Ducati from Honda, and is strongly fancied to win a first world crown since 2019.
The start of the new MotoGP season has been overshadowed by the absence of world champion Jorge Martin.
The Spaniard suffered fractures to his right hand and left foot in a horror crash during pre-season testing in Malaysia.
Then his Aprilia team said this week that Martin suffered another mishap in training and had to have more surgery.
Aprilia now say he will miss both Buriram and the next race, in Argentina in mid-March.