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He was Cassius Marcellus Clay then, of course — an Olympic gold medalist, undefeated challenger for the world heavyweight crown and the most charismatic (and, for some, the most controversial) athlete of the era. It was February 1964, and the 22-year-old Clay, whose professional record was an impressive 19-0, was slated to fight Sonny Liston for the heavyweight title in Miami Beach. Hardly anyone in the boxing world — and certainly very, very few in the close-knit and deeply conservative boxing media — gave the supremely self-confident Kentuckian the slightest chance of beating the hard-punching “Big Bear.”
And while no one gave him much of a chance, nor could anyone possibly ignore a fighter who recited his own comical, self-aggrandizing poetry to anyone who would listen. (One pre-fight gem: “The crowd did not dream when they put up the money / that they would see a total eclipse of the Sonny.”)
But on the night of Feb. 25, 1964, Clay took the crown from Liston with a 7th-round technical knockout in a performance that marked the arrival of an unmistakably formidable boxing talent. Clay — soon to be renamed Muhammad Ali — possessed an unprecedented set of skills, including balletic footwork, lightning hand speed and bone-crushing punching power. When the referee counted Liston out in that ring in Miami Beach, it was clear that the world of boxing, and the universe of sports in general, would never be the same.
As the great New York Times sportswriter Robert Lipsyte wrote on the night the fight:
Liz Ronk, who edited this gallery, is the Photo Editor for LIFE.com. Follow her on Twitter at @LizabethRonk.