Fullmer won the world middleweight championship on January 2, 1957, when he upset the legendary Sugar Ray Robinson by soundly winning a unanimous 15-round decision. On May 1, 1957 they fought a rematch. The fight began as expected, with Fullmer using his strength and awkwardness to bull into Robinson and really force him onto his heels. In the fifth round Robinson, while backing up, lashed out with what has been called the perfect left hook. It caught Fullmer flush on the chin and knocked him out.