Oleksandr Usyk was vicious in his first fight as a heavyweight, forcing Chazz Witherspoon to retire after the seventh round.
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The brilliant Usyk's fast hands and feet were a nightmare for the larger Witherspoon in Chicago, and he landed punches at will until his opponent could not continue.
The victory was a textbook debut in the heavyweight division for Usyk, who last year became the undisputed cruiserweight champion by winning the World Boxing Super Series. He immediately signalled his intention to become just the third boxer ever to add a heavyweight world title to cruiserweight gold.
Usyk was nearly two and a half stone lighter than his opponent, and it showed, but it also gave his extra speed with his feet as he glided in and out of range.
His left hand from the southpaw stance began to slip through Witherspoon's guard after a few rounds of finding his timing.
Witherspoon, a 38-year-old who came in at a few days' notice to replace Tyrone Spong, tried to thunder big counter punches and although he did occasionally catch Usyk, he could never discourage him.
Usyk's movement began to drown Witherspoon who just could not keep up. The Ukrainian was able to land shots at will in the sixth round. The referee considered stopping the fight here, but did not act.
In the seventh Usyk hunted for the knockout, landing a barrage of unanswered shots. Witherspoon remained upright but wilted back to his stool, and could not get up.
Usyk retains his status as the mandatory challenger to the WBO heavyweight title that Andy Ruiz Jr will defend against Anthony Joshua on December 7, live on Sky Sports Box Office. Usyk expects to challenge the winner next year.