A former world champion fighter who helped make history this week at the Rio Olympics says a stint in prison a decade ago for robbing a tourist launched his boxing career.

Amnat Ruenroeng of Thailand was among three professional boxers allowed to compete in Brazil -- the first time pros have taken part at an Olympics.

The 36-year-old celebrated by claiming a unanimous points decision on Sunday in his opening lightweight bout and later revealed that he had once been a petty criminal facing a 15-year stretch in a grim Thai jail.

Many might have fallen farther into a life of crime, but Amnat saw it as a chance for redemption and he discovered boxing, eventually representing the jail at tournaments.

"I was in jail for one-and-a-half years until the king pardoned me," he said.

"Life in prison changed me a lot. You get disciplined and you learn how to tell what's wrong and what's right. I received an education for the first time.

"I had the opportunity to try boxing, football and kickboxing. But most importantly prison taught me the importance of family and friends.

"It helped me develop concentration and it gave me a conscience, to think before acting."

Amant, who has a 17-1 professional record and once held the IBF flyweight title, was released from prison in 2007 and a year later was on his way to the Beijing Games.

He narrowly missed out on a medal and afterwards turned professional.