American boxer Patrick Day has died in hospital, four days after suffering a serious brain injury in a fight against Charles Conwell. He was 27.

Key points: Patrick Day suffered a traumatic brain injury after being knocked out by Charles Conwell in Chicago

Patrick Day suffered a traumatic brain injury after being knocked out by Charles Conwell in Chicago Day had fought in 22 professional fights, winning 17 with one draw

Day had fought in 22 professional fights, winning 17 with one draw Conwell released an open letter to his opponent two days before Day passed away

Day was knocked out by Conwell in the 10th round of their super-welterweight (69.9 kg) clash in Chicago on Saturday.

His promoter, Lou DiBella, released a statement saying Day died "surrounded by his family, close friends and members of his boxing team".

Day was carried away from the ring on a stretcher at the conclusion of Saturday's fight.

Conwell released an emotional public letter to his opponent on Tuesday.

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"I never meant for this to happen to you," 21-year-old Conwell wrote.

"All I ever wanted to do was win.

"No one deserves for this to happen to them.

"I can't stop thinking about it … I prayed for you so many times and shedded [sic] so many tears because I couldn't even imagine how my family and friends would feel."

Conwell said he "thought about quitting boxing" but instead would continue, using Day as inspiration.

"I'll use you as motivation every day and make sure I always leave it all in the ring every time."

Conwell said this would be the last time he spoke about the matter, owing to it being "a sensitive topic" for himself, Day's family and "the sport of boxing," but did retweet his letter today.

DiBella's statement said Day knew the inherent risks associated with the sport, but called for authorities to make boxing safer.

"Patrick Day didn't need to box. He came from a good family, he was smart, educated, had good values and had other avenues available to him to earn a living," DiBella said.

"[Day] chose to box, knowing the inherent risks that every fighter faces when he or she walks into a boxing ring.

"Boxing is what Pat loved to do. It's how he inspired people and it was something that made him feel alive."

Day is the latest boxer to die as a result of injuries sustained in the ring, following Maxim Dadashev and Hugo Santillan in July.

"It becomes very difficult to explain away or justify the dangers of boxing at a time like this," DiBella's statement continued.

"This is not a time where edicts or pronouncements are appropriate, or the answers are readily available.

"It is, however, a time for a call to action. While we don't have the answers, we certainly know many of the questions, have the means to answer them, and have the opportunity to respond responsibly and accordingly and make boxing safer for all who participate.

"This is a way we can honour the legacy of Pat Day.

"Many people live much longer than Patrick's 27 years, wondering if they made a difference or positively affected their world.

"This was not the case for Patrick Day when he left us."