Chris Eubank Jr claimed the WBA interim middleweight title after Russian opponent Matt Korobov suffered a shoulder injury in the second round.
On his debut in the United States, Eubank Jr, 30, was declared the winner by a technical knockout in Brooklyn.
Korobov, 36, injured his shoulder after throwing a punch early in round two and was unable to continue.
Eubank Jr dropped down a division for the fight and the bout was his first since he beat James DeGale in February.
In the main fight at the Brooklyn event, unbeaten American Jermall Charlo retained his WBC middleweight title by stopping Irishman Dennis Hogan in the seventh round.
Hogan was dropped with an uppercut in the fourth round and hit the canvas again after a left hook three rounds later.