Hariss Harun scored the only goal of the game as Singapore surprised 2016 finalists Indonesia with Fandi Ahmad's side claiming a 1-0 win in the AFF Suzuki Cup.
Hariss struck after the Indonesian defence failed to clear Gabriel Quak's low cross while Faris Ramli missed two later chances to add to Singapore's lead before all three points in the group B encounter were secured.
Meanwhile, Adisak Kraisorn fired six goals as champions Thailand made an ominous start to the defence of their title on Friday evening with a 7-0 thrashing of East Timor.
Adisak's haul was one short of the record for a single match, held by Singapore's Noh Alam Shah when he netted seven in an 11-0 win over Laos in 2007, as Milovan Rajevac's side outclassed the Timorese in Bangkok.
The Muangthong United striker opened the scoring after just three minutes with a side-footed effort and, 10 minutes later, he doubled his side's lead with a perfectly weighted lob over goalkeeper Aderito Raul Fernandes.
A simple close range finish gave him his first hat-trick two minutes after the half-hour mark and a header on the stroke of half-time secured Adisak and his team a four-goal advantage.
Five minutes into the second half he started and finished the move that resulted in Thailand's fifth before scoring his sixth from the penalty spot in the 57th minute after Nurul Sriyarngem had been brought down by substitute Feliciano Goncalves.
Goalkeeper Fernandes was sent off 17 minutes from time for a foul on Mongkol Tossakai to further compound East Timor's misery and Supachai Jaided added the seventh in injury time.
The biennial 10-team tournament continues on Monday when Malaysia host Laos in group A while Myanmar kick off their campaign against Cambodia.
The top two finishers in each five-team group advance to the semifinals, which will be played in early December, with a two-legged final to be played on Dec. 11 and 15.