Thailand were given a penalty kick after 23 minutes which Boon Yong did not make any mistake to take for the only winning goal.
Singapore sought an equalizer, playing roughly and putting Thailand in defensive position.
After 82 minutes Suhaimi had chance to shoot close but Thailand' s keeper Thamsatchanan neatly caught it.
In the final Thailand will play Indonesia whom they beat 4-1 in the group stage while the semi-final losers Singapore and the defending champion Malaysia will jostle for the third place on Saturday.
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