When Joe “The Brown Bomber” won the world heavyweight title, defeating champion James Braddock in June of 1937, he said, “Until I beat Max Schmeling, I won’t be champion!” He was talking about his knockout loss in 1936. He avenged it in 1939 at Madison Square Garden, knocking out Schmeling in the first round.
When “Sugar” Ray Leonard, 27-0, lost to Roberto “Hands of Stone” Duran, 71-1, in a June of 1980 Duran fight, he earned a rematch. He besieged Duran so badly that Duran resigned the chair in November.
In April 1987, Leonard defeated Marvin Hagler to capture the middleweight title. Hagler retired and moved to Italy not wanting a rematch.
In April 2002, lightweight champion Floyd “Money” Mayweather earned a gift decision over Jose Luis Castillo. Eight months later, he gave him a rematch, defeating him easily.
In June 2012, Manny “Pac-Man” Pacquiao was robbed by Tim Bradley Jr., but in April 2014, he defeated Bradley in a rematch.
In November 2016, Andre “SOG” Ward earned a gift decision over Sergey Kovalev in a light heavyweight title bout and immediately gave him a rematch, which resulted in Kovalev being suspended, and he then retired from boxing.
Heavyweight champion Rock Marciano earned a close, if not disputed, decision over Roland Lastarza in March 1950 before winning the title. After stopping “Jersey” Joe Walcott for the title and again in his next fight, he stopped Lastarza.
In November 1986, super featherweight champion Alfredo Escalera won a worst split decision win over Philadelphia's Tyrone Everett in Philadelphia. He never got a rematch, and the Pennsylvania judge who voted out Escalera never worked on another show.
Historian Jim Jacobs told me the worst decision he ever saw was in June 1963, when light heavyweight champion Harold Johnson was robbed by Willie Pastrano by split decision and never got a rematch.
Heavyweight champion Muhammad Ali lost to Ken Norton and Smokin' Joe Frazier in their first fights but defeated them twice in their rematches.
In June of 1983, heavyweight champion Larry “The Easton Assassin” Holmes, 42-0, earned a decision gift in a defense against “The Terrible” Tim Witherspoon, 15-0, never giving him a rematch.
What fight did you want to see in a rematch that never happened?