While he didn't score a knockdown, his power did leave an impression as Usyk's continued pressure proved to be too much for Witherspoon. After seven rounds of punishment, Witherspoon's corner told the referee their fighter could no longer continue, much to the delight of the 9,073 fans inside Wintrust Arena.

"I was waiting for this moment, we had some little difficulties because we have to change opponents," Usyk said after the fight. "I didn’t show it to nobody but I had some bad thoughts float into my head. I just prayed a little bit, I asked (God) to help me, and it looks like He helped me."

Usyk took his time early on to gauge what Witherspoon would bring to the table, but the late replacement for Tyrone Spong didn't have much to offer beyond being a warm body for the man who was returning to the ring after an 11-month layoff and beginning his journey to try to join Evander Holyfield as the only fighters to become undisputed cruiserweight and heavyweight champion.

Usyk (17-0, 13 KOs) started to pick up the pace in the third round as he began pounding Witherspoon with a blistering right jab that allowed the 2012 Olympic gold medalist to find a home for a straight left hand. 

Usyk's power started to round into form in the fourth, which is when he began breaking down Witherspoon. By the end of the fifth round, Usyk's combination punching had taken its toll on Witherspoon and left him fatigued and worn down. It was only a matter of time before the Ukrainian put the listless Witherspoon out of his misery. Any time Witherspoon tried to throw anything of substance, Usyk used slick footwork to move out of the way and blast Witherspoon with precise counterpunching.

Witherspoon's fairytale week finally ended in the seventh. Usyk uncorked a wicked assortment of punches with about 90 seconds remaining. If it weren't for the ropes, Witherspoon would have gone down. When the round came to an end, Witherspoon's head trainer, Tommy Brooks, told referee Hector Afu that he was stopping the fight.

What is next for the 32-year-old Usyk is unknown. He's the mandatory challenger for the WBO title since he was the undisputed champion at cruiserweight before relinquishing his belts, which sets him up for the winner of the Dec. 7 rematch between Andy Ruiz Jr. and Anthony Joshua. Whether it's the winner of that fight or WBC titlist Deontay Wilder, Usyk says he is ready.

"I am ready to fight whoever," Usyk said. "If they give me the mandatory, then, of course, I am ready to take it."