Her baby girl's cries of distress on the other side of the locked door told her that something was horribly wrong - but she was powerless to stop her from being raped.
The 24-year-old mother pounded on her locked bedroom door and begged the young man inside the room with her five-month-old baby to let her in.
But the alleged rapist, a close friend, insisted that the key was jammed in the lock.
In desperation the woman - who may not be identified to protect her child's identity - ran to a nearby public telephone to call the police. But she was too late.
She returned "three minutes later" to find the door open and her child crying. The baby had been raped and was so severely injured that she later underwent emergency surgery.
The suspect had fled after a security guard in the building allegedly tipped him off about the telephone call to police.
The five-month-old girl is the latest victim of a series of brutal baby rapes.
The first case to rock the nation was that of Baby Tshepang, a nine-month-old girl from Louisvaleweg in the Northern Cape who was gang-raped and horrifically injured last month by six men.
A second case, a week ago, involved the abduction and rape of a sleeping eight-month-old girl from her home in Ravensmead, Cape Town. The badly injured child was left lying in a dark street, where she was found by passers-by.
The latest incident happened around 5pm in Joubert Park, central Johannesburg, where the mother lived.
"I had heard about the baby who was raped in the Northern Cape," she said. "But I never believed such a thing could happen until my child was raped."
On Saturday night, doctors performed a colostomy on the little girl, a procedure where a hole is made in the abdominal wall to provide an artificial anus.
It is unclear if the baby will have to live with a colostomy bag, which is inserted into the hole to collect faeces, for the rest of her life.
She was in a stable condition in Johannesburg Hospital. She was wearing an oxygen mask and being fed by a drip.
Her mother has been unable to sleep since the incident happened and has remained by her side, standing next to the beeping heart monitor alongside the cot.
"It's not nice to cry in front of the patients. But we are shocked," said Sister Emma Nhlapho, who learnt about the rape when she came on duty on Sunday morning.
The mother said she cried so much after the incident, when she realised her baby was bleeding, that she could barely carry her baby. A stranger took pity on her and accompanied her to Johannesburg Hospital, where they were taken by police.
She said the father was "heartbroken. He even dropped the telephone."
Members of the Family Violence, Child Protection and Sexual Offences Unit on Sunday afternoon arrested a suspect, thought to be 17 years old.
Police had their hands full trying to apprehend the suspect. They patrolled the streets of Joubert Park while the suspect called them from various public telephones before they finally persuaded him to hand himself over.
A second, 24-year-old suspect, whose connection with the case was unclear, was being held for questioning, and police were seeking a third person.
Part of the mother's ordeal is knowing that the suspect was a close friend who used to play with her child.
"He even used to say she would be his daughter-in-law when she grows up."