He was reading a newspaper in his 14th-storey flat when he heard a loud crash.
Consultant Y.X. Wong, 30, looked out of the window and saw a motorcyclist lying at the junction of Toa Payoh Lorong 1 and Lorong 4.
The man, who is in his 20s, was injured after his motorcycle collided with a white Honda Civic at about 12.45pm yesterday at the T-junction.
He suffered abrasions to his face and hands and was sent to Tan Tock Seng Hospital for treatment.
Mr Wong said: "The impact of the accident must have been very strong because it sounded very loud in my flat.
"The motorcycle was also about 10m away from the motorcyclist. I think he's very lucky to have survived."
Mr Wong said some motorists and pedestrians went to the motorcyclist's aid immediately after the accident.
But no one dared to touch him, he said. A pedestrian helped to direct traffic away from the victim, while two others appeared to be talking to him.
He added: "The motorcyclist appeared to be in very bad shape and was twitching. It was nice of the motorists and pedestrians to try to help him."
The Singapore Civil Defence Force (SCDF) said it received a call about the accident at about 12.45pm.
The driver of the Honda Civic was not sent to hospital, but an SCDF spokesman could not confirm if the driver was injured.
Police investigations are ongoing.
This article was first published in The New Paper.