Russian boxer Maxim Dadashev has died at the age of 28 following injuries suffered in his IBF light-welterweight fight against Subriel Matias.
Dadashev was reportedly unable to walk to the dressing room after his bout was stopped by trainer Buddy McGirt at the end of the 11th round on Friday.
He was hospitalised with bleeding on the brain and underwent emergency surgery but failed to recover.
The Russian Boxing Federation paid "deep condolences" to his family.
USA-based Dadashev had won all of his previous 13 fights but had to absorb a barrage of punches from Puerto Rican Matias during the course of the fight in Maryland.
McGirt had said afterwards he "could not convince" his fighter to stop, but opted to throw in the towel when he saw him "getting hit with more and more clean shots as the fight went on".
Northern Ireland's former world champion Carl Frampton was among those to pay tribute, saying on Twitter: "Saddened to hear about the passing of Maxim Dadashev. Deepest condolences to his friends and family. RIP."
British boxing promoter Eddie Hearn added: "So terribly sad to hear the news of the passing of Maxim Dadashev. Rest in peace."