Having lost 1-0 to the same opponents in a friendly last year, the Southeast Asian champions went down by the same scoreline as they suffered a third loss on the trot
The Warriors FC custodian was in splendid form throughout but could only watch on when midfielder Bai He pounced on a rebound after he had made yet another good stop to fire in a 60th-minute winner, which was enough to give the hosts victory.
Fresh off a 6-1 hammering by China last Friday, Singapore coach Bernd Stange would have been disappointed at how his team lost their way after a decent start in Mong Kok Stadium.
Qiu Li, restored to the starting line-up, fired an early effort wide before Khairul Amri followed suit a while later.
But the home side gradually got into their stride and Hassan was called upon to deny Jaimes McKee in the 24th minute when the English-born striker got his head to a corner.
Two minutes later, Hassan got a superb touch to Huang Yang's 25-yard piledriver as he tipped the ball onto the bar.
Naturalised Ghanaian striker Godfred Karikari fired just wide as the end of the half approached, with Lions forward Indra Sahdan having to be replaced by Fazrul Nawaz after suffering a knee injury.
Hong Kong did not let up after the break and Hassan had to be alert in the 50th minute to smother a through ball, racing off his line to get to it before Karikari.
He could do nothing about the goal when it came though. Lee Chi Ho's header from a freekick was smartly saved by Hassan, but Bai was on hand to drive home the loose ball with the Singapore defence all over the place.
Fazrul then touched a cross from Shakir Hamzah wide as the Lions looked for a swift response, but Hassan prevented them from falling further behind when he superbly denied McKee in a one-on-one situation.
There was to be no late reprieve for Singapore with Amri failing to get a decisive touch to Baihakki Khaizan's long ball near the end, as they fell to a third consecutive defeat.