The Great White Hype (1996)

When the champ's promoter, Reverend Sultan, decides something new is needed to boost the marketability of the boxing matches, he searches and finds the only man to ever beat the champ. The problem is that he isn't a boxer anymore and he's white. However, once Reverend Sultan convinces him to fight, he goes into heavy training, while the confident champ takes it easy, and falls out of shape.