Who was the first boxer to beat Muhammad Ali in a professional fight?

In 1971, both Ali and Frazier had legitimate claims to the title of World Heavyweight Champion. An undefeated Ali had won the title from Sonny Liston in Miami Beach in 1964, and successfully defended his belt up until he had it stripped by boxing authorities for refusing induction into the armed forces in 1967. In Ali's absence, the undefeated Frazier garnered two championship belts through knockouts of Buster Mathis and Jimmy Ellis. He was recognized by boxing authorities as the World Champion. Unlike Mathis and Ellis, Frazier was plausibly Ali's superior, which created a tremendous amount of hype and anticipation for a match pitting the two undefeated fighters against one another to decide who was the true heavyweight champ.

The Fight of the Century is the title boxing writers and historians have given to the boxing match between WBC/WBA heavyweight champion Joe Frazier (26–0, 23 KOs) and Ring magazine/lineal heavyweight champion Muhammad Ali (31–0, 25 KOs), held on Monday, March 8, 1971, at Madison Square Garden in New York City. Frazier won in 15 rounds retaining the title with a unanimous decision, dealing Ali his first professional loss.