Oleksandr Usyk stops Chazz Witherspoon in round seven to win heavyweight debut
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Oleksandr Usyk used his speed to trouble the heavier Chazz Witherspoon
Oleksandr Usyk won his first fight at heavyweight after stopping Chazz Witherspoon in round seven in Chicago.
It means Ukrainian Usyk, 32, remains the WBO's mandatory challenger to the winner of Anthony Joshua's heavyweight-title rematch with Andy Ruiz Jr.
Usyk had not fought since he defended all four world cruiserweight titles against Tony Bellew in November 2018
Witherspoon stood in with four days' notice after original opponent Tyrone Spong returned an adverse doping test.
Usyk was asked if he was prepared to fight the likes of Joshua, Ruiz and WBC heavyweight champion Deontay Wilder next.
"This shows I am ready to fight them," said the 2012 Olympic champion.
"Of course if they give me this fight, I will take it."
Wilder will defend his belt in a rematch against Luis Ortiz in Las Vegas on 23 November, while Ruiz will defend his IBF, WBO and WBA titles against Britain's Joshua in Diriyah, Saudi Arabia on 7 December.
Usyk started watchfully against 38-year-old Witherspoon before he started to get through the American's defences with a series of forceful blows to the body.
He came close to forcing a stoppage in the sixth but Witherspoon held on.
Usyk searched for a knock-out in the seventh, backing Witherspoon into the corners and landing with a flurry of punches and, although the American got back to his stool after the round ended, he could not get up again.
It extends Usyk's unbeaten professional record to 17 wins, 13 by knock-out.
He had been due to make his heavyweight debut against Carlos Takam in May but had to pull out because of a biceps injury.