Thailand lost to South Korea 1-0 in a must-win match for the War Elephants in the AFC U23 Asian Cup in Uzbekistan on Wednesday.
Go Jay-hyeon scored the lone goal for the defending champions 35 minitues into the game to stay atop of Group C.
Thailand have four points and South Korea have seven to stay top of Group C.
In another Group C match, Vietnam beat Malaysia 2-0 to stay second in the group.