He heard a commotion and looked up from his mobile phone to see a man on the ground in a pool of blood.
Mr Mohamed Nazir Abdul Rahiman, 28, then saw a woman clinging to the shirt of another man who was trying to get away.
The auditor with accounting firm KPMG tackled the man and pinned him down with an armlock at a grass patch outside UOB Plaza in Raffles Place.
The incident, which happened around 1pm yesterday, lasted for all of five minutes.
When he spoke to The New Paper, Mr Nazir's clothes were covered in blood, which he said came from the alleged attacker during the struggle.
He was on his way to a mosque for Friday prayers when he heard a woman screaming for help.
"I saw a Caucasian lady on her knees helping a man on the ground," said Mr Nazir.
"I thought I had stumbled on a Channel 8 drama but when I saw the man bleeding profusely, I realised that something was wrong." "The woman was trying to stop a second man from running away, but she wasn't strong enough to hold him."
He said that he just lunged forward on instinct and pinned down the man. The alleged attacker, believed to be in his 40s, was at least 1.6m tall and spoke in accented English, he added.
"It didn't occur to me that there could have been a weapon around and I could have got injured," said Mr Nazir.
"I wanted to help the woman because other people just looked on as she was trying to hold on to the man. I did not want him to get away."
An eyewitness told TNP that he had seen the suspect stab the victim twice near the exit of Raffles Place MRT station before fleeing with the victim's laptop bag.
Once Mr Nazir had the man in an armlock and pressed his knee on the man's back to prevent him from moving, he called the police on his phone.
"I applied pressure on him using tactics I learnt when I was a policeman (during national service)," he said.
"He shouted and struggled and seemed to be in pain because he was injured, but I didn't let go."
Other people helped Mr Nazir to keep the alleged attacker on the ground until the police arrived about five minutes later.
After the suspect was arrested, Mr Nazir left the scene.
The area was teeming with people as it was lunchtime, and many of them were shocked by the brazen attack in broad daylight.
The attacker was believed to have been hurt during his struggle with the victim. A knife was recovered from the scene.
The police said they received a call about the attack at 1.04pm. Officers later determined it was a case of armed robbery with hurt.
One of the two men taken to Singapore General Hospital has been arrested and investigations are ongoing.
The victim is believed to be badly wounded.
Mr Nazir said he didn't think his actions were heroic.
"My colleagues asked me why my trousers was covered in blood but I didn't say anything, I told them they can read the news to find out," he said with a laugh.
"I was only trying to help. The real heroes are the women who tended to the victim who was bleeding.
"What they did probably helped to save his life."