A source close to Frazier told the New York Post that the 67-year-old, whose professional career lasted from mostly from 1965-76, is "in serious shape" and that "we're looking for a miracle."
Frazier was the champion in the late 1960s and early 1970s. He fought Muhammad Ali in three classic heavyweight fights, winning once and losing twice.
One of the most memorable fights for Frazier came in 1975 against Ali in what's been called the "Thrilla in Manila." In that October 1 bout, Frazier's trainer, Eddie Futch, stepped in to stop the fight after the 14th round with Ali clearly ahead on points. That fight was for the WBC and WBA heavyweight titles.
Frazier's next fight, in June of the following year, ended in a fifth-round technical knockout loss to George Foreman.
That would be the final fight for Frazier, until he attempted a brief comeback try in December 1981 when he had a 10-round draw against Floyd "Jumbo" Cummings.
A member of the International Boxing Hall Of Fame, Frazier finished his professional career with a record of 32-4-1 with 27 knockouts. He also won a gold medal at the 1964 Olympics in Tokyo.
The first fight against Ali was a classic, at Madison Square Garden on March 8, 1971. Frazier won a spectacular 15-round decision for the WBA and WBC championship. Ali then won a 12-round unanimous decision over Frazier in 1974.