It emerged over the weekend Frazier, 67, is fighting liver cancer and he is reported to be in hospice care.
Ali, 69, said on Monday: "The news about Joe is hard to believe and even harder to accept.
"Joe is a fighter and a champion and I am praying he is fighting now.
"My family and I are keeping Joe and his family in our daily prayers. Joe has a lot of friends pulling for him - and I'm one of them."
Frazier beat Ali on points in the so-called 'Fight of the Century' in 1971, Ali's first professional defeat.
But in two further fights, including the epic 'Thrilla in Manila' in 1975, it was Ali who was victorious.
The pair have had a fractious relationship over the years, stemming from the Ali's taunting of Frazier in the run-up to their famous trilogy of fights.
Ali called his rival a "gorilla" and "Uncle Tom", taunts which Frazier found difficult to forgive, though the pair were reported to have been on better terms in recent years.