The defending champions, who are struggling to put together the team without a title sponsor, did not get the services of their ace Nigerian forward Ranty Martins because of suspension.
But the side under stand-in coach Anjan Nath who took charge after Dutchman Eelco Schattorie, rose to the occasion as Baldeep Singh (21th) and Eric Brown (35th) scored one each to set up a semi-final clash against Mohammedan Sporting.
Deepak Mondal was instrumental in both the goals as he first set up Baldeep with a measured centre before setting up the second goal as Brown converted it from a close range.
Geylang went all out for an equaliser in the second-half, but United SC held on to their slender lead.