The security guard saw the tree branch fall and could hear it crashing to the ground.
While he hoped no one was hurt, what he saw next shocked him - a woman lying under the branch, a large wound on the right side of her head.
Mr Avtar Singh was directing traffic outside the Hyflux Innovation Centre (HIC) at Bendemeer Road when the accident occurred at about 9.30am yesterday.
He was about 20m away from the tree when he heard the sound of leaves rustling and looked up just in time to see the branch crash down.
The branch, which was about 5m long, fell on Madam Cao Yanyan, an employee of IT company NEC, which is at the HIC building.
She was believed to have been walking in the direction of Serangoon Road.
Madam Cao was found lying on the pavement with a large wound on her head. The impact appeared to have left her dazed, and she was staring into space, said Mr Singh.
"The right side of her face was swollen and there was blood all over it,"he added.
By then, a few motorists in the vicinity had pulled over and got out to help her.
The security guard added that he did not understand what they were saying as they were speaking in Mandarin.