Boxer Patrick Day has died at the age of 27 after sustaining head injuries in his fight in Chicago on Saturday, promoter Lou DiBella has confirmed.
Day was knocked out in the 10th round of his bout against Charles Conwell and subsequently underwent brain surgery.
DiBella said in a statement on his website: "Patrick Day passed away today, succumbing to the traumatic brain injury he suffered in his fight this past Saturday, October 12, at the Wintrust Arena in Chicago, IL.
"He was surrounded by his family, close friends and members of his boxing team, including his mentor, friend and trainer Joe Higgins."
Conwell, 21, wrote an emotional letter to Day following the fight.
"I never meant this to happen to you, all I wanted to do was win," he said.
"If I could take it all back, I would. No-one deserves this to happen to them."
Day won 17 of his 22 fights, with four defeats and one draw.
In July, Russia's Maxim Dadashev and Argentina's Hugo Santillan both died following injuries sustained during fights.