The Lions threw on top scorer Ilhan at half-time for Kweh, with Anuar moving to the right.
There was no let-up from Thailand after the break, extending their lead in the 50th minute through Seksan.
A loose pass from Tan was cut out by the Thai midfield, before Patrik Gustavsson – who had come on for Teerasak in the first half – outpaced Tan and crossed for Seksan, who finished cleanly.
Singapore coach Gavin Lee brought on Ryhan Stewart for Tan in the 56th minute.
Ilhan stole in behind the Thai defence in the 59th minute and nearly rounded Kampol before he was called back for offside.
The Lions had more of the possession, but the final ball was lacking at most times and the Thai defence was hardly threatened.
Thailand threatened Singapore on the counter, and in the 64th minute, Izwan was called into action with a good save when one-on-one against Picha Autra.
In the 83rd minute, Ilhan curled a shot into the bottom right corner from just outside the box, drawing loud cheers from the crowd.
Singapore pressed for another goal. Despite the late rally, the Lions failed to test Kampol properly and the Thais ran out winners.
