She was making out a receipt to a customer when she saw a man collapse outside the bicycle shop.
Miss Jeannie Toh, 25, who works at the Mighty Velo shop in Lavender Street, quickly called the Singapore Civil Defence Force (SCDF) and shouted for her colleague to alert her boss.
The incident happened at around 4.50pm on March 12.
She picked up a calendar and rushed out to check on the man, who seemed to be in his 50s.
She said: "I saw his eyes turning blood-shot and he wasn't blinking. I tried to fan him with the calendar.
"His breathing was shallow, and I called out 'Uncle, Uncle', but he didn't respond."
Miss Toh had attended a one-day cardiopulmonary resuscitation (CPR) course about seven years ago when she was a combat signaller in the Singapore Armed Forces.
She took the man's pulse by feeling the area on his neck near the chin with two fingers.
"I didn't feel any pulse. But I didn't know if it was because he had no pulse or because I was shivering," she said.
SCDF Sergeant Ramli Selamat, 31, was the one who picked up her call. He asked her to describe the situation and despatched an ambulance immediately.
He asked to hear the man's breathing over the phone, so Miss Toh put the phone on speaker mode and held it against the man's mouth.
From her account and from the man's shallow breathing, Sergeant Ramli assessed that the man was having a heart attack.