ALI VS LISTON, FEBRUARY 25, 1964
Muhammad Ali was 22 and still known as Cassius Clay when he took on heavyweight world champion Sonny Liston for the first time in Miami Beach, Florida. The brash, fast-talking challenger taunted Liston unceasingly in the build-up to the bout, but few expected him to win.

Clay came out strong, using speed and footwork to his advantage against the slower Liston. After the sixth round, Liston, who was suffering from cuts and bruises under his eyes and an apparent injured shoulder, announced he couldn't continue. Clay won the match by technical knockout and then proclaimed to the world: "I am the greatest!"

ALI VS FOREMAN, OCTOBER 30, 1974 
Ali became the second fighter ever, after Floyd Patterson, to regain the heavyweight world title when he took on and knocked out George Foreman in the eighth round of the Rumble in the Jungle in Kinshasa, Zaire in 1974.

Ali came out dancing, and Foreman, feared for his punching power and sheer physical presence, went right at him. Early in the second round, Ali went to the ropes and covered up, letting Foreman swing away -- later dubbing the strategy the "rope a dope".

Foreman spent his energy throwing punches in the sweltering heat, egged on by taunts from Ali. Ali staggered Foreman with a combination early in the fourth and again in the fifth -- when the champion was weary.

In the eighth, Ali landed a final combination, a left hook that pulled Foreman's head up so Ali could nail him with a hard right that sent Foreman staggering back and down. He couldn't rise before the count.

ALI VS FRAZIER, OCTOBER 1, 1975 
Ali was in a lighthearted mood in the build-up to the third installment of his rivalry with Frazier, who was thought to be washed up after a devastating loss to Foreman.

But Ali's taunting of Frazier as a "ugly, dumb gorilla" and "White Man's Champion" infuriated Frazier, who trained with grim intensity for what was billed as the Thrilla in Manilla.

When Ali came out fast with a flurry of combinations, Frazier pushed through the punishment and as Ali tired, Frazier stepped his attack with damaging left hooks.

Frazier dominated the middle rounds, but began to tire in the 10th and Ali turned the tide.

In the 11th, Ali connected with a series of speedy combinations that left Frazier's eyes all but swollen shut. 

Frazier's trainer Eddie Futch stopped the fight between the 14th and 15th rounds, over the objections of Frazier, who was hailed by Ali as "the greatest fighter in the world -- next to me."