Both are in Group B along with Vietnam, Malaysia, and Laos.
The Thail team are hoping to claim their 17th title in the event, after losing the title to Vietnam in the last two editions of SEA Games.
Thailand put the pressure on from the eighth minute when Singapore failed to deal with a cross from the right. A subsequent flick-on at the near post was then turned into a goal by Teerasak Poeiphimai.
The War Elephants’ second goal came soon after when Achitpol Keereerom finished off a move inside the box in the 38th minute after some neat interplay from the Thais.
Singapore gave themselves a lifeline three minutes later when Nicky Melvin Singh made the most of a howler from Thailand goalkeeper Soponwit Rakyart, who dropped the ball in the box.
Five minutes into the second half, Purachet Thodsanit tucked in a cut-back from right-back Phongsakon Trisat. Having restored their two-goal advantage, the Thais then switched to defensive mode until full time.
The War Elephants will play their second Group B game against Malaysia on Saturday (May 6) at the same venue.