Maxim Dadashev suffered a stroke during his fatal with with Subriel Matias but fought on.
Dadashev was pulled out of the fight by his trainer Buddy McGirt at the end of the 11th round.
The 28-year-old collapsed as he left the ring and was rushed to hospital where part of his skull was removed to ease the pressure on his brain.
But efforts to save the dad-of-one proved in vain and he passed away on Tuesday.
His wife Elizaveta Apushkina was on holiday at the time but rushed from Turkey to Washington to be by his bedside.
And she revealed her husband's surgeon said he had suffered a stroke during the bout.
"Do you want to know what is true grit, strength of mind and the highest will for victory?," she said.
"What it means to be a warrior?
"Watch this fight and when you ask yourself how Max, with his defence skills got hit with so many punches, know that he had already suffered a stroke, according to doctor who operated on him.
"And he continued the fight feeling horrific, unbearable pain, losing orientation.
"But he did not give up.
"He rose up from his knees a million times, went forward and never gave up."
Relatives of the boxer are planning to bring his body back to Russia.
“There are currently no obstacles for sending the body to Russia,” said a source.
His remains would be buried at Peterhof, near St Petersburg, where he lived.
