Former undisputed cruiserweight champion Oleksandr Usyk 17-0 (13) had little trouble with Chazz Witherspoon 38-4 (29) in his heavyweight debut, stopping him in seven one-sided rounds at the Wintrust Arena in Chicago on Saturday night.
Witherspoon stepped in at short notice to replace Tyrone Spong, who had failed a drugs test.
Ukrainian southpaw Usyk, 32, was cautious early on but upped his workrate in the middle rounds before forcing Witherspoon to retire on his stool at the end of the seventh.
The six-foot-three Usyk weighed in at 215-pounds for his first fight at heavyweight.
“I believe I will add more weight for my future fights,” said Usyk at the post-fight press conference. “Coming in heavier isn’t a problem but I want to do it the right way, to make sure I don’t lose any of my speed.
“The difference between cruiserweight and heavyweight is the dynamics. I can feel the power more at heavyweight, due to the (opponents) being heavier.
“I think the best way of avoiding heavyweight power is to defend myself better, not to get caught with those big punches.”
Usyk was installed as the number one heavyweight by the WBO when he moved up in weight and will likely fight for the world title early in the new year.
Andy Ruiz Jr will defend the WBO belt along with his WBA and IBF titles against Anthony Joshua in a rematch in Saudi Arabia in December.
“I’m ready for them,” Usyk said. “If they are going to offer it to me, of course I’m going to take it.”
 had little trouble with Chazz Witherspoon 38-4 (29) in his heavyweight debut, stopping him in seven one-sided rounds at the Wintrust Arena in Chicago on Saturday night.){kind=link}