Bus driver Zhang Kun said yesterday that he had slammed on his brakes - seconds before the vehicle ploughed through a central divider, killing a passenger.
But for some reason, they did not work, he told The Straits Times.
"The bus suddenly went crazy, and hurtled towards the road barrier," the 39-year-old Chinese national said in Mandarin.
"I didn't know what would happen," he added.
Sunday's accident left a 19-year-old Malaysian man dead and another passenger injured after the bus veered across four lanes and crashed through the barrier, before flipping onto its side.
When it finally shuddered to a halt, the driver said he found himself trapped in his seat.
From the corner of his eye, he could see the two passengers on board.
"I wanted to reach out to them, but I couldn't," he said. "I was stuck."
Mr Zhang - who has a wife and two children aged six and 17 in his native Henan - added that he was freed from the bus by helpful passers-by, and then taken to the National University Hospital.
He is now awaiting surgery for spinal injuries, and said he is fearful about his fate. He does not know when he will be discharged, or whether he will still be allowed to drive.
"I feel really terrible. I'm in low spirits," he told The Straits Times from his hospital bed.
"I didn't mean to harm anyone. I feel very guilty towards the passenger who lost his life."
Mr Zhang, who was nursing a fever and had an ice pack on his forehead, said the day had started "just like any other".
"It was an ordinary day, and the road conditions were good."
But he said his brakes failed as he tried to slow down when exiting the slip road from the Bukit Timah Expressway.
His employer, SMRT, said he had been driving the same No.700 route for the past year, and had a clean record, with no prior complaints against him.
A spokesman added that Mr Zhang had "more than the required eight-hour rest period between his shifts" from Saturday to Sunday.
Yesterday, the transport firm released a statement saying it was working closely with the police on their investigation. "Our hearts are with the families of those affected," it added.
"We are doing our best to support and assist the injured and family of the deceased."
Singapore Road Safety Council chairman Bernard Tay said he was saddened by the accident. He urged public transport operators to take all necessary measures to make preventing accidents on the road "a prominent part" of their operations. "Passengers using our buses should always feel safe," he said. "It is the last place they should expect to lose their life."