Charles Conwell has written a letter of tribute to Patrick Day as he fights for his life after the pair's bout in Chicago.
The boxer was knocked out by Conwell in the 10th round on Saturday night, fighting at super-welterweight on the undercard of Oleksandr Usyk's heavyweight debut against Chazz Witherspoon.
Conwell landed a right hand early in the 10th which caused Day to stumble and seconds later Conwell landed a left hook that sent Day to the canvas. He was instantly knocked out and it was not clear if he regained consciousness.
Day was treated by a doctor in the ring and then rushed off on a stretcher by paramedics and transported to Northwestern Memorial Hospital, where he suffered a seizure and is now in intensive care after having emergency surgery.
"I never meant this to happen to you, all I wanted to do was win," Conwell wrote. "If I could take it all back, I would. No-one deserves this to happen to them.
"I replay the fight over and over in my head thinking what if this never happened and why did it happen to you.
"I can't stop thinking about it myself. I prayed for you so many times and shed so many tears because I couldn't even imagine how my family and friends would feel.
"I see you everywhere you go and all I hear is wonderful things about you. I thought about quitting boxing but I know that's not what you would want.
"I know that you were a fighter at heart so I decided not to, and to fight and win a world title because that's what you wanted and that's what I want, so I will use you as motivation every day and make sure I leave it all in the ring every time."
Conwell also said this would be the last time he spoke on the matter because it was "a sensitive topic" for Day's family and friends, himself and the sport of boxing.