Light heavyweight Roman Simakov, who was carried from the ring on a stretcher after being knocked out in a WBC Asian Boxing Council title fight on Monday, has died, the Russian federation said.
The 27-year-old had brain surgery after being rushed to hospital in the Ural city of Yekaterinburg but never regained consciousness and died on Thursday.
"I can only say that the investigation into his death is taking place," RBF official Igor Maruzov was quoted as saying by local media.
Simakov was knocked down in the sixth round by Russian compatriot Sergei Kovalev but got up and was able to continue before receiving numerous blows to the head in the seventh when he was knocked out.
"My aim was to outbox Simakov, not necessarily knock him out," Kovalev was quoted as saying by Russian media on Thursday.
"But after the fourth round, I had noticed something was wrong with Simakov. My last punch was not as powerful as the one I had hit just before that."
It was Simakov's first knockout defeat and only his second loss in 21 fights during his professional career.