This actor was a professional boxer and he knocked out a man who was at one time a top five heavyweight contender in a real match at a major American boxing venue. This man fought for a state boxing championship. *another clue--this actor probably competed with Jay Silverheels for roles in movies. If you couldn't get Silverheels, you probably tried to get this man in the 1950's and 1960's.
It was THE HARDER THEY FALL, but this actor was not a star like Rod Steiger. And gad, I can't see Steiger as a boxer.
Another clue--this boxer turned actor was a City Golden Gloves champion. He fought boxers who in turn fought Marciano, Moore, and Patterson.