Sergey Kovalev defended his WBO light-heavyweight title with a seventh-round stoppage of Cedric Agnew in Atlantic City.
The Russian floored Agnew in the second and sixth rounds before ending the contest with a left to the liver early in the seventh.
Agnew dropped onto his knee in pain and remained there long after referee Samuel Viruet had counted him out.
Kovalev, whose record improves to 24-0-1 including 22 knockouts, did not have it all his own way at the Boardwalk Hall, suffering a cut above each eye from a headbutt in the fourth and elbow in the sixth.
"I tried boxing because he has good defence," Kovalev said. "Then I went to the body. I saw how hurt he was."
Agnew had come into the fight boasting an unbeaten 26-fight start to his professional career but spent most of the night on the ropes, deflecting punishment before launching the occasional counter-attack.
The Chicago fighter enjoyed occasional success with left hooks and body shots but eventually found himself overwhelmed.
"He has an uncomfortable style," Kovalev added. "He's not stupid. He kept a good distance. He was patient and I understood he was trying to tire me out, but I was ready."
Kovalev, who took his title off Welshman Nathan Cleverly last year, refused to discuss a potential light-heavyweight unification showdown with WBC champion Adonis Stevenson. The pair are contracted to rival television stations.
He said: "I don't want to speak on Adonis Stevenson."