A male security guard on duty at Suntec City performed oral sex on a drunk man after finding him collapsed on the floor of a toilet.
A court heard that Low Chih Kin, 35, took advantage of the 36-year-old lying motionless after a heavy drinking session.
Low was sentenced to six years' jail and three strokes of the cane yesterday after pleading guilty to one count of performing a non-consensual sexual act.
Two molestation charges involving the same man were taken into consideration during sentencing.
The court heard that the victim - an assistant building manager with an asset management firm - had been drinking with colleagues at the mall and convention centre on May 6 last year.
He went to a toilet on the ground floor at about 1am and ended up lying on the floor, where he remained motionless. His wife called him at around 2.45am as he still had not returned home.
Deputy Public Prosecutor Eunice Lau said: "The victim, who was still lying on the toilet floor at the time, was able to make out that his handphone was ringing, and surmised that his wife must have been trying to reach him.
"However, due to his level of intoxication, the victim was unable to open his eyes - let alone move his hand to answer his handphone - to respond to the calls."
Low went to the toilet about 15 minutes later when he saw the man lying on his side at its entrance. Feeling sexually aroused, he performed oral sex on the man, who was unable to open his eyes or fight off the security guard.
However, he could feel Low committing the offence, the court heard.
Low later stopped and a closed circuit television (CCTV) camera caught him leaving the area.
The victim, who cannot be named for legal reasons, finally answered his wife's call at around 3.15am and she rushed to Suntec City to help him.
Despite his condition, the man managed to tell her that he had been sexually assaulted.
She alerted the police and her husband was taken to Tan Tock Seng Hospital. The court heard that Low was traced through CCTV footage and arrested.
For committing the non-consensual sexual act on the man, he could have been jailed for up to 20 years and fined or caned.