At the age of 18 Cassius Clay (second from right) with the gold medal in the light-heavyweight division at the summer Olympics in Rome. "I was completely exhausted and just wanted the round to end," his opponent in the gold-medal bout, Zbigniew Pietrzykowski, whose face was covered in blood by the third round, later recalled. Clay turned pro later that same year.

When Muhammad Ali first won the heavyweight title, he ran around the ring shouting "I am the greatest." Although many called him a loudmouth at the time, later nobody would dispute that he had earned the title he chose for himself, "the greatest boxer of all time."

Ali faced Liston in the first defense of his title. He made short work of his much older opponent, sending Liston to the canvas just two minutes into the first round. "Get up you bum," Ali bellowed. The fight was stopped shortly afterwards.

Clay was outspoken ahead of his first shot at the world heavyweight title in 1964, when he faced Sonny Liston. The challenger promised to knock out the champ, whom he described as a "fat bear" in the eighth round. It took him just six. Liston's corner threw in the towel, saying he was suffering from a shoulder injury. Later that same year, Clay changed his name to Muhammad Ali.

Ali described his bout against West German boxer Karl Mildenberger in Frankfurt's Waldstadion in 1966, which was his sixth defense of his world title, as the toughest since he became champion. Ali won after the referee stopped the fight in the 12th round due to a cut over one of Mildenberger's eyes. Ali said he never wanted to fight the West German again.

The first defeat of Ali's professional career came against Joe Frazier at Madison Square Garden in New York in 1971. Frazier knocked Ali down in the 15th round and won the fight in a unanimous decision. The two boxers earned what was then a record $2.5 million for what was dubbed the "fight of the century."

In 1974, Ali sought to win back the title by facing the champion, George Foreman in Kinshasa. Ali went into the bout as the underdog, as Foreman was one of the hardest punchers the sport has ever seen. Ali danced around for seven rounds taking punch after punch from the champion, taunting him the whole time.

Then suddenly, Ali burst into action, sending Foreman to the canvas with a lightning fast combination of punches. The "rumble in the jungle" was over after eight rounds. Ali had his belt back and became just the second heavyweight to regain the title.

Many boxing experts regard Ali's fight against Joe Frazier in the Philippines in 1975 as the greatest contest ever. Both boxers tested their physical limitations in the "thrilla in Manilla." The referee stopped the fight with Ali leading on points in the 14th round because one of Frazier's eyes was so swollen that he was having trouble defending against Ali's punches.

In 1976 Ali tarnished his reputation somewhat by agreeing to take part in a bizarre fight against Japanese professional wrestler Antonio Inoki. Inoki spent most of the time lying on the canvas and trying to kick at Ali's legs. Ali spent most of his time trying to reach down to punch him. The strange 15-round spectacle ended in a draw.

In 1978 Ali made the mistake of not taking his opponent, Leon Spinks seriously. Overweight and having not trained very hard, Ali lost his title again. In a rematch later the same year, Ali became heavyweight champion for a third time and announced his retirement. His later comeback was unsuccessful.

After his career, Ali would become friends with two of his toughest opponents in the ring, Joe Frazier and George Foreman.