Boxing legend Muhammad Ali has reached out to his former rival Joe Frazier following his cancer diagnosis, revealing he is "praying" the former World Heavyweight Champion fights off the disease.

The boxer, nicknamed Smokin' Joe, is battling liver cancer and has been receiving treatment at a hospice in Philadelphia since last week.

Frazier became the first man to beat Ali as a professional when he knocked him down in a 1971 match, and now the retired fighter has paid tribute to his former opponent by extending his sympathies to the star and his family.

"The news about Joe is hard to believe and even harder to accept," Ali tolds CBS in a statement. "Joe is a fighter and a champion and I am praying he is fighting now. Joe has a lot of friends pulling for him, and I'm one of them."

He also revealed he is giving Frazier's family "daily prayers".