Michael Coffie was unexpectedly knocked out by late-notice opponent Jonnie Rice on Saturday night in New Jersey.
Coffie was an undefeated threat in the heavyweight division but must now rebuild after a shock loss.
He had earned a step up to fight former world title challenger Gerald Washington after impressively beating Darmani Rock. But Washington withdrew earlier in the week after testing positive for Covid-19 opening the door for Rice to steal the show.
A barrage of unanswered right hands from Rice in the fifth round forced the referee to step in and rescue Coffie.
Rice had previously lost six of 15 fights, including against Efe Ajagba, Demsey McKean, Tony Yoka and Arslanbek Makhmudov.
"I didn't expect to overwhelm Coffie like that," said Rice. "I knew Michael Coffie from his fights, but not because he had a great record but because he was a sparring partner for Deontay Wilder.
"In my head, I played the little game that I was Luis Ortiz's sparring partner for his second fight against Wilder. So technically it was Luis Ortiz's vs Deontay Wilder's sparring partners. We're going to see what happens. I was playing with that in my head.
"I had to fight a little bit like Deontay Wilder and then a little bit different. That's what the game plan was.
"I wasn't a late replacement. I was waiting for an opportunity. We replaced someone, but we weren't late. We were right on time. I was training every day. I was training with Michael Hunter. I did 20 rounds, 10 and 10 last Friday before I got the call. So when I got the call, I was ready."
"I want it all. I want what every boxer wants. I want to be world champion. I understand that my record doesn't say that, but I want the world to watch my progress."