Thailand beat Singapore 3-1 in Bangkok on Jun. 11 to seal the Lions' fate for the 2026 World Cup qualifying campaign.
However, the result was not enough for the Thais as they narrowly missed out on qualifying for the World Cup despite the win on Tuesday.
Thailand needed to win by three goals to leapfrog China into second place in Group C.
Forward Ikhsan Fandi scored the only goal for the Lions at Rajamangala National Stadium to dash Thailand's hopes.
China squeezed through after a 1-0 defeat to South Korea in the other Group C match earlier on Tuesday evening.
Thailand's goals
Midfielder Suphanat Mueanta tapped in a cross from Chanathip Songkrasin in the 36th minute.
The half ended 1-0 to Thailand.
Ihksan Fandi equalised for Singapore against the run of play in the 56th minute, even as the Thais had continued to press forward.
In the 78th minute, substitute Poramet Arjvirai fired a shot past Singapore's goalkeeper Hassan Sunny.
Fellow substitute Jaroensak Wonggorn headed from close range in the 85th minute.
Scored 5, conceded 24
Singapore ended their group-stage campaign with one point, after no wins, a draw and five losses.
The Lions scored five goals and conceded 24 in total after six games.
They lost their first two matches in their qualifying group in November 2023, falling 3-1 to Thailand and 5-0 to South Korea in Seoul.
Head coach Tsutomu Ogura's first match in charge saw Singapore hold China to a 2-2 draw, before losing 1-4 in Tianjin.
Singapore got routed 0-7 at home by South Korea before this Thailand match.
Top photos via Football Association of Singapore Facebook