HARI SREENIVASAN: Fight night – Miami Beach – February 1964

ANNOUNCER: Clay's jab is stronger than it has been at any point in the fight

HARI SREENIVASAN: Clay is a decided underdog, but he pounds the champ into submission in just six rounds.

ANNOUNCER: At the end of this round, Liston's corner will call the doctor to the ring and – over the champion's protest – stop the fight.

HARI SREENIVASAN: Now, Clay is boxing's new heavyweight champion.

ANNOUNCER: Clay is proclaiming 'I am the greatest … I am the king.

HARI SREENIVASAN: Just that quickly, a unique figure emerges in American sport, and will go on to become a global icon.

MUHAMMAD ALI: I am just like oxygen – all over the world.

HARI SREENIVASAN: Born January 17, 1942, Cassius Marcellus Clay, Junior, grew up in Louisville, Kentucky. He learned to fight at an early age, seeking vengeance for a stolen bicycle. And he piled up awards as a young boxer.

In the 1960 Rome Olympics, Clay stopped a Polish fighter to take the light-heavyweight gold. But returning home to segregated Louisville, he was denied service at a whites-only restaurant and threw his Olympic medal into the Ohio River.

Despite that frustration, Clay landed a sponsorship deal the same year and won 19 straight professional bouts. The perfect mark earned him the shot at Liston for the heavyweight title, at just 22 years old.

MUHAMMAD ALI: I whooped him so bad he had to go to the hospital. And I'm still pretty. Whatcha gonna say about that? Huh?!"

HARI SREENIVASAN: Clay was already known as "The Louisville Lip" for his outlandish self-promotion, even writing lyrics about himself. As Rock Newman recalled…

ROCK NEWMAN: This is the legend of Cassius Clay, the most beautiful fighter in the world today. He talks a great deal, and brags indeed-y, of a muscular punch that's incredibly speed-y. This kid fights great; he's got speed and endurance, but if you sign to fight him, increase your insurance. Ah, rumble, young man, rumble.

HARI SREENIVASAN: Newman is an ex-boxing promoter who went on to host a public TV talk show in Washington.

ROCK NEWMAN: He was so physically gifted, with blinding, lightning-like speed for a heavyweight fighter. He did most things wrong technically in the ring, but he could get away with it because of his blinding speed and his superior reflexes.

HARI SREENIVASAN: Clay basked in the boxing spotlight. But he was also undergoing momentous changes outside the ring. Earlier, he had met Malcolm X, the black nationalist leader, and at the time, spokesman for the nation of Islam.

After the Liston fight, Clay officially joined "the nation and changed his name to Cassius "X." Soon, the group's leader, Elijah Muhammad, renamed him again as "Muhammad Ali."

REPORTERS: Cassius? Cassius?

MUHAMMAD ALI: You know my new name, why do you keep calling me that?

REPORTER: Will your next fight be billed as Cassius Clay or as Muhammad Ali?

MUHAMMAD ALI: Muhammad Ali!

REPORTER: On all the fights?

MUHAMMAD ALI: Yes sir!"

HARI SREENIVASAN: Ali's religious conversion and outspoken views made him a lightning rod in the turmoil of 1960's America. But his boxing dominance continued. In 1965, he faced off against Liston again, in Lewiston, Maine. This time, the fight lasted less than two minutes. As Liston lay on the mat, Ali stood over him, taunting him, in what became an iconic image. Questions swirled about whether Liston threw the fight, but Ali waved aside all doubts, with what became his signature phrase.

MUHAMMAD ALI: Look at that beautiful face, fella. You've never seen a man in history move like this. Ain't that beautiful? Float like a butterfly, sting like a bee, that's what I'm saying."

HARI SREENIVASAN: Then, as Ali entered his prime, the escalating war in Vietnam confronted him with an entirely different opponent – the United States government. He was drafted in 1967, but claimed conscientious objector status.

MUHAMMAD ALI: Why should me and other so-called negroes go 10,000 miles away from here in America to drop bombs and bullets on other innocent brown people who've never bothered us? I will say directly, no I will not go.

HARI SREENIVASAN: The Justice Department ruled the objection was political, not religious and Ali was stripped of his title, and did not fight again for three and a-half years. He also faced a potential prison term, but remained free on appeal…

MUHAMMAD ALI: I don't worry about jail. I believe in Allah. I believe in Elijah Muhammad as the Messenger of God and many great men have to go to jail and so I don't pay no attention to it. If the time comes, I'll just have to go.

HARI SREENIVASAN: By 1970, with the anti-war movement at a crescendo, boxing authorities allowed Ali to return to the ring. That set up a match with the man who'd claimed the heavyweight title in his absence – Joe Frazier. Their bout at Madison Square Garden in march 1971 was billed as "the fight of the century." But the long layoff had robbed ali of his speed…

ANNOUNCER: Muhammad Ali has never taken such a battering.

HARI SREENIVASAN: Frazier kept the title after 15 grueling rounds. Within months, though, Ali scored a major legal victory when the u-s supreme court upheld his conscientious objector claim, and wiped away his prison sentence. Freed of all obstacles, he launched a rise back to boxing prominence.

ANNOUNCER: Rumble in the jungle.

HARI SREENIVASAN: Culminating in Kinshasa, Zaire, 1974, and the fight dubbed "the rumble in the jungle." Ali faced the younger, hard-hitting George Foreman, who had beaten Frazier.

ANNOUNCER: Round 1 – the heavyweight championship of the world at stake.

HARI SREENIVASAN: This time, he used a strategy he named "rope-a-dope" to wear out Foreman.

ANNOUNCER: The punches aren't doing any damage, though.

HARI SREENIVASAN: It worked. Late in the 8th round, Ali landed a combination that sent foreman to the mat, and once again, he was champion.

MUHAMMAD ALI: I told you, all of my critics, I told you all that I was the greatest of all time when I beat Sonny Liston. I told you today I'm still the greatest of all time.

HARI SREENIVASAN: Less than a year later in the Philippines Ali was back in the ring with Frazier for the rematch called the "Thrilla in Manila." This time, the champion took a beating, but finally won on a technical knockout in 14 rounds. After that, Ali lost the heavyweight championship to Leon Spinks in February 1978, then reclaimed it one more time, before losing his final fight in 1981. He retired from boxing at the age of 39 with a record of 56-and-5. Poet and author Nikki Giovanni knew Ali well.

NIKKI GIOVANNI: Ali was not a politician, he had no ambition in that way. he was an athlete and he shown all the athletes – he was an athlete, who said, 'no, it doesn't matter what you all think about me or what you say. It doesn't matter your praise. I need to stand for something.' and he's done that.

HARI SREENIVASAN: Three years into retirement, Ali revealed he had Parkinson's disease. But he stayed active, despite his symptoms.

MUHAMMAD ALI: Thank all of you for your support and following me over the years in boxing.

HARI SREENIVASAN: In 1990, he visited Iraq to help win the release of 14 u-s hostages from Saddam Hussein. Six years later, he took center stage once again, lighting the Olympic flame at the summer games in Atlanta, amid a sea of flashbulbs. And in 2005, President George W. Bush awarded him the medal of freedom. Even in old age, Muhammad Ali remained a larger-than-life figure As the subject of movies and documentaries, commercials and posters. These days, his legacy lives on, in gyms around the country, among young fighters and their trainers.

LAMONT PETERSON, FORMER WORLD JUNIOR WELTERWEIGHT CHAMPION: First thing your coaches tell you about him and start looking at his skill and to try to pick up things in the ring, but there was more to Ali more than just boxing – people love him and make movies of him it's for a reason

BARRY HUNTER, FORMER TRAINER FOR LAMONT PETERSON: The name itself is synonymous with boxing. Ali – boxing. Boxing – Ali. And I doubt very seriously in our lifetime that we will ever see another one like him.