A contentious knockdown has proved costly for Joseph Parker in his unanimous-points defeat to Dillian Whyte at the O2 Arena on Sunday.
The Kiwi heavyweight was coasting in the first two rounds, picking apart his British opponent, before he was sent to the canvas right before the bell after a clash of heads.
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Whyte leaned in with his shoulder, dropping his head and colliding with Parker's temple, prompting referee Ian John-Lewis to rule it a knockdown.
Parker seemed visibly shaken and struggled to sustain his earlier dominance, allowing Whyte to take over the fight, landing several heavy blows.
Parker's trainer, Kevin Barry, said the incident had lasting effects on his fighter.
"I think it was a round we were winning and it went from being a 10-9 round to us to a 10-8 round for Dillian.
"He was actually a little dazed from that and shocked when he came back to the corner.
"I actually think that the effects of that headbutt were suffered in Rounds Three and Four. Joe just wasn’t himself and it wasn’t until the fifth round, I believe, he had everything going again."
Parker would hit the deck once more in the ninth round, but finished strongly, dropping Whyte in the final minute of the bout.
While the headbutt knockdown undoubtedly changed the course of the fight, it's affect on the judges' scorecards was decisive.
Whyte was given the nod 113-112, 115-110, 114-111. If the knockdown had been called a head clash, the fight almost certainly would have been a draw.
A serious case could also be made that Whyte should have had points deducted for constant leaning in on Parker and shots below the waist.
John-Lewis warned Whyte on several occasions, but that's as far as he went, refusing to penalise the Brit's tactics.
Parker struggled to cope with his opponent's brawling style, with openings become less prevalent as the fight went on.
The Kiwi found success going to the body, but tired and couldn't sustain his technician-like approach.
Parker showed tremendous heart in the final two rounds, despite being up against it, but perhaps suffered from the rub of the green.
Instead, it's his second loss on the bounce, following his defeat to Anthony Joshua at Principality Stadium in April.
That loss was also not without contention, after Italian referee Giuseppe Quartarone came under fire for constantly separating the pair, when exchanges got too close for his liking.
Another Parker defeat, another fight marred by controversy.