Muhammad Ali has said he is praying for Joe Frazier following the news of his great rival's battle against cancer.
It was revealed over the weekend that Frazier, 67, who fought Ali in three famous bouts during his career, is in hospice care with liver cancer.
Ali, 69, said: "The news about Joe is hard to believe and even harder to accept. Joe has a lot of friends pulling for him - and I'm one of them.
"Joe is a fighter and a champion and I am praying he is fighting now."
The former world heavyweight champion added: "My family and I are keeping Joe and his family in our daily prayers."
In the first of their encounters in the ring - the so-called 'Fight of the Century' in 1971, Frazier inflicted a first defeat on Ali courtesy of a points decision.
In two further fights, including the 'Thriller in Manila' in 1975, it was Ali who was victorious.
The pair had a fractious relationship during their fighting days, stemming primarily from Ali's taunting of Frazier in the build-up to their trilogy of fights.
However, the pair are thought to have been on better terms in recent years.
Former heavyweight boxer Larry Holmes also gave his support to former sparing partner Frazier.
"If you talk about heart, determination, courage, all that good stuff, you're talking about Joe Frazier," said Holmes. "I got my fingers crossed.
"I've been watching the news all day and all night, trying to find out what's happening with him. I know it don't look good, but if anybody can beat what he's up against, you'd have to think Joe would be the guy."
Frazier held the world title himself between 1970 and 1973, claiming the belt by defeating Jimmy Ellis in New York before relinquishing it to George Foreman.