Ironman competitors rushed to perform CPR on a man who crashed to the ground, bloodying his face, after he came off his bike during the event in Auckland.
The man, aged in his 40s, was taken to hospital in a critical condition. He was in a serious but stable condition yesterday.
Event organisers said they did not know if the man had crashed and suffered injuries, or if he had suffered a medical condition such as a heart attack, causing him to crash.
Denwin Hanvey was watching the race and rushed over to the man when he realised he had come off his bike on Constellation Drive, on Auckland's North Shore.
He did not see the crash happen, but believed the man could have suffered a medical event before coming off the bike.
"He had a lot of blood on his face when we arrived," Mr Hanvey said.
"Three other cyclists stopped who were doctors or nurses or specialists."
He said paramedics arrived quickly in an ambulance.
"The paramedics didn't say anything to us, they were just trying to keep him alive."
Mr Hanvey said he had competed in other Ironman events and triathlons and had seen bike crashes, but never something as serious as this.
A competitor, who wanted to remain anonymous, said he stopped to assist the fallen man.
He said CPR was required for a period of time.
St John paramedics and bystanders did an "outstanding job", he said.
"I think this man was very lucky."
The gruelling half-ironman event, part of a worldwide series, involved a 90km bike ride, a 1.9km swim and a 21.1km run.