Joe Frazier, left, knocks Muhammad Ali off balance with a powerful left hook.
Ali sits up looking for revenge after taking a hard hit by Frazier.
Ali, against the ropes, struggles to escape the corner as Frazier throws a left jab.
Ali tries to throw a punch as Frazier overpowers him with a strong left.
Frazier stands over Ali as he lies on the canvas in pain.
Ali, defeated, lies on the canvas at Madison Square Garden after Frazier knocks him down for the win in the 15th round.
A referee directs Frazier to his corner after knocking down Ali -- fallen for only the third time in his career -- early in the 15th round.
Frazier triumphantly raises his glove after beating Ali by unanimous decision, keeping his heavyweight title.
Tuesday marks 40 years since the then-heavyweight champion Joe Frazier met the yet-undefeated Muhammad Ali in the ring. In the "Fight of the Century," held in 1971 at New York's Madison Square Garden, Frazier beat Ali in a 15-round fight with a unanimous decision.