Sonny Bill Williams says an undetected heart condition accompanied him throughout his sporting career, including premiership-winning campaigns with the Bulldogs and Roosters.
The 41-year-old former NRL and All Blacks star recently underwent open-heart surgery at Melbourne's Austin Hospital.
Surgeons replaced his aortic valve and reconstructed a second valve, an operation Williams feared he might not survive.
In a social media video posted Friday night, he described the procedure as his greatest challenge.
"That's the hardest thing I've ever done," Williams said.
"Day two, honestly … I thought I wasn't going to make it," he said.
Williams said he had boxed Paul Gallen 15 months earlier without knowing the condition could have caused him to die suddenly.
He recalled a doctor telling him: "it's a miracle that you found it because it's a case where a perfectly fit-looking person could drop dead".