Patrick Day, a 27-year-old American boxer, is "fighting for his life," his trainer Joe Higgins said according to USA Today.
The super welterweight was dominated in a Saturday bout against a 21-year-old called Charles Conwell.
In Chicago's Wintrust Arena, Conwell knocked Day down in the fourth and eighth rounds and then out in the 10th.
Day left the ring on a stretcher and had seizures en route to hospital.
A statement released by the promoter's DiBella Entertainment said the athlete was in "extremely critical condition" as of Sunday.
Patrick Day is currently "fighting for his life" after he suffered a knockout defeat and was hospitalized on Saturday.
Day, a 27-year-old American, was competing against Charles Conwell in Chicago's Wintrust Arena but was dominated by his 21-year-old opponent.
Conwell knocked Day down in the fourth and eighth rounds with hard right hands before knocking him out in the 10th. Day received medical attention in the ring, left on a stretcher, and had seizures en route to hospital.
"Patrick Day suffered a traumatic brain injury during his bout on Saturday night. He was rushed to Northwestern Memorial hospital where he underwent emergency surgery," the promotional company DiBella Entertainment said in a statement.
"As of Sunday evening, Patrick is in a coma caused by the injury and is in extremely critical condition. On behalf of Patrick's team, we appreciate the outpouring of support, prayers, and offers of assistance from all corners of the boxing community."
Day's trainer Joe Higgins said: "He's fighting for his life," according to USA Today.
Conwell added: "My thoughts and deepest prayers are with Patrick Day and his family right now."