It was one of the biggest upsets in heavyweight history.

On March 31, 1973, a then-unheralded Ken Norton scored a stunning 12-round split decision win over Muhammad Ali at the San Diego Sports Arena, breaking the great ex-champion’s jaw, and, for many, ending any hopes he had of regaining the title.

Norton, best known for being a Joe Frazier sparring partner, was a 29-1 contender. However, there was nothing on the ex-marine’s resume to suggest that he had the beating of Ali, who was on a 10-fight win streak following his first loss, to Frazier, in March 1971.

Ali had come in heavy at 221 pounds. He had developed a nasty habit of training up to the level of competition and it cost him against Norton. The Adonis-like Californian, who was trained by Eddie Futch, was the stuff of nightmares stylistically. He could match Ali’s jab – a rarity – and his cross-armed defense proved difficult for the former champ to penetrate.

When Ali’s jaw was broken in this bout has been the subject of great conjecture. The Ali camp insisted it happened in the second round, a session that his opponent won with some explosive power punching. Team Norton, however, were convinced that Ali’s jaw was fully broken in the 12th and final round, which the underdog thoroughly dominated. There is evidence on the original ABC broadcast that Ali was struggling earlier than that.

Either way, Norton did the damage with punches and this night belonged to him.