His decline was tragic - before his death at the age of 51, Mathis had survived two strokes, a heart attack and heart failure. Yet go back three decades and Mathis was a brilliant heavyweight prospect. He stood 6ft 31/2in tall, and, although he was on the bulky side at around 18st, was a clever, crafty boxer with surprising speed for such a big man.
In 1964 he won the US Olympic trials by beating Joe Frazier twice, but a broken hand forced him to withdraw - and Frazier took his place at the Tokyo games, returning with the gold medal and a perfect launching pad for a professional career. While Frazier hogged any publicity given to rising stars, Mathis learnt his trade thoroughly under his manager Cus D'Amato, compiling a record of 23 consecutive wins, including a third- round stoppage of Chuck Wepner.
Muhammad Ali was stripped of the world heavyweight crown in 1967 because of his refusal to be drafted for the Vietnam War and the boxing world argued over who should be his successor. While the World Boxing Association put together an elimination tournament that was eventually won by Ali's sparring partner Jimmy Ellis, the New York State Athletic Commission matched Frazier and Mathis for their version of the championship.
And in Madison Square Garden, New York, in March 1968, Frazier knocked out Mathis in the 11th round to begin the reign that eventually brought him to the marvellous 15-round points win over Ali in 1971. For the loser, Mathis, there was only more of the treadmill. But he won six fights in a row, including a 12-round decision over the Canadian brawler George Chuvalo. Then Jerry Quarry outpointed him and he retired.
With his wife Joan and baby son Buster Jnr, Mathis lived on what was left of his ring earnings, then came back and fought Ali in November 1971. He went the full 12 rounds in a contest for the North American championship. He finally quit for good in September 1972 when Ron Lyle, one of the biggest- punching fighters of his generation, knocked him out in two rounds.
"It was hard," Mathis said. "I thought it would be easy. I'd be a fighter instead of getting a job." Mathis was not an educated man. He dropped out of school as early as he could, and it was only when he was an adult that he was found to be dyslexic. But nobody had a bad word to say about him, which in the end matters more. An example of his kindness came before the Ali fight. No big hotels for him; he stayed at a local Travel Lodge. One day he saw the owner's daughter selling candy for her Brownie pack. He bought some, then took the rest into the lobby and persuaded rather startled guests to buy up every piece the little girl had.
After Mathis retired his weight ballooned to more than 36st. "Food was my weakness, my downfall," he said. "For some people it's booze or drugs. For me, it's always been food."
Mathis admitted he was stunned to learn that his son, Buster Jnr, wanted to be a fighter. In 1989 the boy boxed in the National Golden Gloves, but Buster Snr was too ill to go to watch. "If he makes it in boxing, I'll be pleased," he said. "I'll be more pleased if he stays in school and gets his degree." Buster Jnr did that, majoring in maths and communications at Aquinas College, in Grand Rapids.
Young Buster's decision to fight gave Mathis an interest in the sport once more and, even though sometimes he needed a walking frame, he taught amateurs at the Pride Boxing Gym in Grand Rapids. In September 1993, someone organised a tribute dinner for him - and 700 attended, including his old rivals Frazier and Chuvalo. Some of the money raised went to Buster Mathis, but more went to the Kidney Foundation of West Michigan. That's the way he wanted it.
In an ironic postscript, Buster Mathis Jnr fights Mike Tyson at the MGM Grand Garden in Las Vegas on 4 November. Buster Jnr's middle name is D'Amato, given him after the legendary manager who guided his father in the 1960s. It was D'Amato who, two decades later, discovered and nurtured Tyson.
Buster Mathis, boxer: born Sledge, Mississippi 5 June 1944; married (one son, one daughter); died Grand Rapids, Michigan 7 September 1995.