In the 78th minute, substitute Poramet Arjvirai cut in from the left and fired a shot past Hassan, putting Thailand back in the lead.
Ihksan missed a golden opportunity to equalise less than two minutes later, heading wide of the goal from a Faris Ramli cross.
Fellow substitute Jaroensak Wonggorn ensured a frantic finish with a header from close range in the 85th minute.
Singapore’s Nur Adam Abdullah had a chance to pull one back for the Lions in extra time but his chip from inside the box was cleared off the line.
Jaroensak fired wide in the 95th minute in what was the last chance of the match, which meant China took second spot in the group.