MANCHESTER, England — Undisputed world cruiserweight champion Oleksandr Usyk successfully defended his four belts and likely sent Tony Bellew into retirement by knocking out the British fighter in the eighth round in Manchester on Saturday.

Usyk sent Bellew to the canvas following a combination that ended with a looping left. The referee got to a count of six before calling off the fight.

It was the Ukrainian fighter’s 16th straight win as a professional and his first defense since becoming the unified champion. He could now move up to heavyweight.

The 35-year-old Bellew, who had said this would be his last fight, had the better of Usyk early in the fight but began to get worn down by the sixth round.

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