Brian McClair's youngsters started the game immediately on the front foot, with James Weir hitting the post after just 45 seconds following a neat one-two with Sam Byrne.
Byrne himself had a great opportunity to open the scoring just three minutes later as he fired a shot straight at Singapore keeper Rudy Khairullah after good work by Jack Barmby.
A competitive, but ultimately goalless, first half ensued with Singapore captain Raihan Rahman stinging the palms of Pierluigi Gollini from distance, while Barmby's free-kick forced a decent save from Rudy.
Singapore's best opportunity came right on the stroke of half time, as Declan Dalley did extremely well to block Shahfiq Ghani's shot after the 20-year-old forward had been slipped through by Syafiq Zainal.
United suffered a nightmare start to the second half, however, as Dalley’s challenge felled Sahil Suhaimi, who dusted himself down before coolly sending Gollini the wrong way from the penalty spot on 52 minutes.
The Reds responded well and again Jack Barmby was involved - his curling shot floated just over the crossbar as United pressed forward for an equaliser.
It eventually came as Barmby and Mats Daehli combined from a quick corner, and James Weir was on hand to flick the Norwegian's low cross home from inside the six-yard box despite the best efforts of Shakir Hamzah on the goalline.