American boxer Patrick Day is in an "extremely critical condition" after suffering a traumatic brain injury in his super welterweight fight with Charles Conwell in Chicago on Saturday.

Day was rushed to Northwestern Memorial Hospital after he was knocked out by his compatriot in the 10th round of a bout on the undercard for Oleksandr Usyk's heavyweight debut against Chazz Witherspoon.

The 27-year-old, who was removed from the ring on a stretcher, is in a coma following emergency surgery.

Day's promoter Lou DiBella tweeted: "Pat Day makes any room he is in a better place. I've never met anyone who's met Patrick and not liked him. Never heard him utter a mean word.

"Never saw him greet someone without a big smile. Life doesn't seem fair sometimes. Please keep Pat in your prayers, thoughts, and hearts."

Conwell said after the fight: "I give my blessings and concerns to him and his team. I hope he is okay. I know he is a great person."

British promoter Eddie Hearn tweeted: "Our thoughts and deepest prayers are with Patrick Day and his family right now.

"We have been in constant communication with his promoter who will provide further information at the appropriate time. Please keep Pat in your thoughts."