Thanakorn handed Chainat the lead in the 22nd minute as his sweetly struck free kick beat Kosin in the Army goal.
Bjorn Lindemann had a shot tipped around the post and the German also clipped the bar from a corner as the home side pressed for an equalizer before the interval.
In the second half, both keepers had to be alert to make crucial saves. Sarayoot was denied on 63 minutes when Panupong made a great save with his right foot after good build up play from the Army. About five minutes later, Kosin got his fingers to a Michael Byrne drive to knock it past the post to keep the Bangkok side in the contest.
With time running out, Army brought on their Argentinian striker, Matius, and within minutes the big man levelled as he beat the offside trap to notch his first goal in Army colors.