Doctors found Sonny Bill Williams' congenital heart defect during a health assessment after neck surgery, despite the 41-year-old having played his entire sporting career unaware of it.
Williams said he had undergone open-heart surgery and feared he might not survive the procedure.
"It's the hardest thing I've ever done. Day two, honestly, I thought I wasn't going to make it. It was crazy," the former All Blacks player said in a YouTube video.
He said doctors told him his heart measured 7.5 centimetres and could have led to him "dropping dead."
"The doctors both said that it's a miracle that you found it because it's a case of a perfectly fit looking person dropping dead."
Williams said the surgery was successful and his heart now worked more efficiently. He described the operation as "two open-heart surgeries in one" and said doctors had reduced the risk of sudden death.
He is expected to recover well and said he would work to raise awareness about heart checks.
Williams won NRL premierships with the Bulldogs and Roosters, represented the All Blacks 58 times and claimed World Cups in 2011 and 2015.
He began his NRL career with the Bulldogs in 2004, winning a premiership in his first season, before switching to rugby union in 2008.
Williams later returned to the NRL and won with the Roosters in 2013. He retired from both codes in 2021.