After recovering from a slow start to the campaign, the Stags had gone on a three-game winning streak which included an impressive victory over Home United, a trashing of derby rivals Geylang United and a come-from-behind win over Gombak United.
And while they were favourites for Thursday's game at the Jurong East Stadium, it certainly wasn't going to be easy against an Albirex side that ended a run of four games without a win last Friday with a 2-1 win over Hougang United.
That eventually proved to be the case as the White Swans grabbed all three points after Yasuhiro Yamakoshi netted the only goal of the game in the 63rd minute.
In Thursday's other game, Home United became the first team to inflict a loss on leaders DPMM FC as they recorded a hard-fought 1-0 victory at the Jalan Besar Stadium.
Frederic Mendy opened the scoring in the third minute when he pounced on a mistake by Shahrulrizal Abd Rahman and slotted past Wardun Yussof.
However, DPMM hit back in the second half and dominated proceedings although they struggled to find a way past the Protectors' stubborn defence.
And even on the solitary occasion they did manage to do so, Home keeper Lionel Lewis came to his side's rescue, tipping over Basiru Osman's powerful header with a fingertip save to ensure his side held on to all three points.
Thursday's Great Eastern-YEO's S.League Results
Albirex Niigata 1-0 Tampines Rovers