Deontay Wilder was spotted using an illegal MMA move in the closing stages of his fight with Joseph Parker.
Wilder, 38, was beaten on points by Parker in the penultimate fight on the 'Day of Reckoning' card in Saudi Arabia.
The former WBC heavyweight champion was dominated from start to finish by Parker, who took the win 118-111, 118-110, 120-108 on the judges' scorecards.
Wilder looked a shadow of his former self and struggled to land any meaningful shots on Parker.
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In the final round of the fight, the American tried to up his pace and attempted to draw Parker into a brawl.
At one point, he landed a number of hammer punches to the back of Parker's head during a clinch.
The punches, which are commonly used in MMA - albeit not the back of the head, are illegal in boxing.
Wilder was warned by the referee for using them but a point wasn't taken from the American.
Fans on social media were quick to call him out for his tactic.
"Deontay Wilder in there throwing hammer fists going out sad," posted one X user.
Another wrote: "Wilders thrown that hammer punch to the back of the head a few times the cheating t***."
"Those hammer fists Wilder throwing are illegal lol," added a third.
While a fourth commented: "Wilder throwing those hammer shots to the back of his head, not on that."
Wilder was expected to next face Anthony Joshua, provided the British fighter beats Otto Wallin.
But defeat for the American casts doubt over that proposed bout.
It has been suggested that Wilder may retire after losing Parker but the former world champion stopped short of hanging up his gloves in his post-fight interview.
"We'll see what happens, we still have a little bit left, but I did a great job managing my money," said Wilder.
"I'm a happy fighter. I'll be back, and if not, then it's been a pleasure. I don't know [if I still have the same fire I used to], a lot has calmed me down.
"I've been wearing this smile all week long and I'mma still wear it. Nothing can stop me on that."
Topics: Deontay Wilder, Joseph Parker, Football, Boxing
Rory O'Callaghan