Charles Conwell has vowed to use Patrick Day as motivation to become world champion as his fellow American fights for his life after suffering a traumatic brain injury in their bout on Saturday.

Day was knocked out by Conwell in the 10th round of the super welterweight fight in Chicago and remains in an "extremely critical condition" in Northwestern Memorial Hospital following emergency surgery on Sunday.

Conwell has written a poignant letter to his fellow American, stating it will be the last time he comments on the situation out of respect for Day's family and friends, himself and the sport of boxing.

The 21-year-old posted on Twitter: "I never meant for this to happen to you. All I ever wanted to do was win. If I could take it all back I would no one deserves for 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 shedded so many tears because I couldn't even imagine how my family and friends would feel.

"I see you everywhere I 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, but to fight and win a world title because that's what you wanted and that's what I want, so I'll use you as motivation every day and make sure I always leave it all in the ring every time. #ChampPatrickDay With compassion, Charles Conwell."