Patrick Day was brutally knocked out by Charles Conwell before being taken to hospital in a harrowing series of events in the knockout round of Oleksandr Usyk's heavyweight debut.
In the final round of what was an exciting fight, Conwell gave two huge punches to Day's jaw, knocking him to the screen.
Day had been knocked over in rounds four and eight, but still seemed to be able to defend and retaliate.
However, in the tenth round, the New York fighter was caught with a crushed left hook before a straight right hand threw him against the screen.
When Day's head collided with the ground, the referee wasted no time in immediately waving to the fight, bringing paramedics into the ring.
The Freeport [New York] boxer stood motionless on his back in a harrowing scene while Conwell was visibly upset in the corner.
Despite securing an impressive and dominant victory, 21-year-old Conwell did not celebrate and did not seem to be taking his eyes off the defeated opponent.
He was seen talking to him and barely smiled when the referee raised his arm while MC host David Diamante made a moderate knockout announcement.
The first knockout was the result of a straight right hand from nowhere, with the second coming from a combination of stings, ending in another straight right.
And it was the same punch that ended the fight, just a minute and 20 seconds before the final bell.
DAZN's Claudia Trejos provided a disturbing update on Day's condition during the broadcast, confirming that he suffered a seizure and did not wake up inside the arena.
"I had a chance to talk to the doctor, he told us that Patrick never woke up on his way to the ambulance.
“He began to learn on the way to the ambulance and was taken to the hospital.
"Our thoughts and prayers are with Patrick Day and his family."
Boxing Scene's Jake Donovan tweeted at 4 am: "They were asked to tweet this message from Patrick Day's handlers – prayers are appreciated, but besides, they're still trying to get in touch with their family, most of which is so far in the dark.
"More information will be released at the appropriate time."
The fight took place in the knockout round of Oleksandr Usyk's heavyweight debut against Chazz Witherspoon.
The former unified cross-king is gaining weight for the first time as a pro, having won all possible belts at the lowest weight.