Claressa Shields tasted defeat in her second MMA fight on Wednesday night against Abigail Montes and was immediately criticised by Jake Paul.
Shields lost for the first time since she was beaten by Savannah Marshall as an amateur when she was out-wrestled by Montes, ending up on the wrong end of a split decision.
The decorated and feted Shields had impressively won her mixed martial arts debut earlier this year but has now felt an upset defeat ahead of her return to the boxing ring on December 11, live on Sky Sports, alongside Marshall.
Shields and Marshall are then expected to contest the biggest-ever women's boxing match next year.
But after Shields was dragged to the canvas on Wednesday night under the Professional Fighters League promotion, she was mocked by YouTuber-turned-boxer Paul. Shields had previously said it would "disrespect" for a boxer of her calibre to ever feature on a Paul undercard.
He called her defeat "karma" and added: "Shields is a loser. In MMA and even more so with her prima-donna attitude.
"The fake always gets exposed.
"Losses come when anger and hate take over. Just ask Ronda Rousey and Conor McGregor.
"She spent more time berating Jake Paul than learning jiu-jitsu."
Shields, 24 hours earlier, had said: "There are a lot of women in boxing who could [out-box] men, especially a regular joe like Jake Paul!
"I know it's too much to fight him because it would be too embarrassing [for him], but if I were to spar Jake, I could use one hand.
"Just my jab - I would out-jab, out-hook and out-uppercut him. I would embarrass him."
Shields had not lost any sort of contest since she was 17 and was beaten by Marshall, the origin of their ongoing rivalry.
The American has since won consecutive Olympic gold medals, world championships in three weight divisions, and become undisputed twice.
Marshall holds the WBO middleweight title and is also unbeaten as a pro.
They will both return to the ring in separate fights on December 11 in the UK before they collide next year.
Sky Sports Boxing schedule
October 31 - Top Rank in New York
Jose Zepeda vs Josue Vargas
November 6 - Top Rank in Las Vegas
Mikaela Mayer vs Maiva Hamadouche - IBF and WBO super-featherweight titles
November 6 - BOXXER in Liverpool
November 20 - BOXXER in London
Richard Riakporhe vs Olanrewaju Durodola
Caroline Dubois professional debut
Hosea Burton vs Dan Azziz - British light-heavyweight title
November 20 - Top Rank in Las Vegas
Terence Crawford vs Shawn Porter - WBO welterweight title
December 11 - BOXXER
December 11 - Top Rank in Las Vegas
Vasiliy Lomachenko vs Richard Commey
December 17 - Top Rank in Montreal
Artur Beterbiev vs Marcus Browne - IBF and WBC light-heavyweight titles
February 26 - Top Rank in Glasgow
Josh Taylor vs Jack Catterall - undisputed super-lightweight title