Claressa Shields claims she inflicted a beating on Savannah Marshall in a never-before-revealed sparring session that lit their rivalry alight.
Marshall returns to the ring on Saturday in Newcastle, live on Sky Sports, knowing victory will propel her closer to the biggest women's boxing match of all time against the feted Shields.
Their history dates back to 2012 when Marshall won an amateur bout between them - it is the only fight, amateur or pro, that Shields has ever lost in a career of stunning achievement.
They are now both signed to Sky Sports Boxing and BOXXER in a commitment to deliver a grudge match which would become the pinnacle of female fights.
Shields has now revealed details for the first time about a sparring session four years later, in the build-up to the 2016 Olympics, where she exacted a measure of revenge on Marshall.
"I whupped her ass," Shields tells Sky Sports. "I made her face as orange as my hair!
"She doesn't speak about that.
"She knows. Ask her about it.
"'It's only sparring?'"
Shields had won Olympic gold in 2012 as a 17-year-old then, after this tale of sparring Marshall, sensationally repeated the feat at the 2016 Games.
Marshall represented Team GB at both of those Olympics.
Since turning pro, Marshall now owns the WBO middleweight title while Shields is a three-division champion and a two-division undisputed champion.
But she is animated when recalling training at the US Olympic training centre in Colorado with her British rival in the same gym.
"She's the only person to beat me so, if we get in the ring for sparring, she's supposed to show that, even though I'm the Olympic gold medallist, she's better," Shields says.
"She has not been better than me since I was 17. I am now 26.
"That's why she hasn't accomplished what I have accomplished.
"The sparring session was simple. I had my hands down the entire time. She could not land a punch on me. I had her face as orange as this hair by the time we were done.
"We were supposed to do two rounds, then switch.
"But I told the coach: 'No. Let her stay here and take this whupping for four rounds. I don't want to switch'.
"I pulverised her for four rounds."
Marshall laughs when Sky Sports ask for her side of this story.
"When has Claressa ever battered anyone? There is your answer," she teases.
So what really happened?
"We went on a two-week training camp to Colorado - Team GB and Team USA," Marshall says.
"The first week? She'd hurt her back. The second week? She hurt her toe. The last day? She was good enough to spar.
"She threw about 10 shots in the whole four rounds and that was it.
"Can you batter someone with just 10 shots?
"I'm surprised she hasn't said that I was on the canvas!"
Shields says that Nicola Adams, Marshall's British team-mate who also won Olympic gold in 2012 and 2016, was in attendance for the spar.
Shakur Stevenson and Charles Conwell, Shields' American team-mates, also watched with interest, she says.
"They said: 'Dang, is that the girl who beat you when you were 17?'
"'Yup'.
"'She can't do nothing with you now'.
"Everybody in the gym said that."
Shields and Marshall had briefly spoken about settling their score a couple of years into their pro careers but it never materialised - several years on, there is no more mouth-watering fight in women's boxing.
"I beat her up in sparring and that's why her team turned down the fight with me in 2018," Shields says.
"The fight will happen and we'll see who has been lying for this entire time."
There have been other painfully awkward moments between Shields and Marshall when their paths have crossed in the nine years since that fateful amateur bout which kickstarted their rivalry.
"I was at a show a couple of years ago," Marshall tells Sky Sports. "And she was there. I thought: 'She's going to come at me'.
"I went over thinking: 'Have you got anything to say?'
"But she said: 'Hi! Are you doing alright?'
"I said a couple of things hoping that she'd bite, but she didn't!"
They also worked out at the Mayweather gym in Las Vegas when Marshall was signed to Floyd Mayweather's promotional company.
"I saw her when she came into the gym," Marshall recalls. "She didn't look at me.
"I said: 'Alright Claressa, how are you doing?'
"She said: 'You look skinny!'
"I thought: 'You're putting me down'."
Shields disputes that: "I saw her at the Mayweather gym. I took a picture with her and her coach."
Shields denies ever displaying animosity: "I was the Team USA captain in 2016. I was nice to everybody. Whenever I saw Savannah I said 'hello'. I have never disrespected her."
The tension has been building for years in this intriguing rivalry.
Shields, a serial winner who calls herself 'the Greatest Woman of All-Time' and her quiet British rival, 'the Silent Assassin', the only person to ever edge her inside the ring.
The clock is ticking down to a grudge match when these backstage whispers can no longer help either of them.
Sky Sports Boxing schedule
October 16 - Top Rank in San Diego
Emanuel Navarette vs Joet Gonzalez - WBO featherweight title
October 16 - BOXXER in Newcastle
Savannah Marshall vs Lolita Muzeya - WBO middleweight title
Hughie Fury vs Christian Hammer
Chris Eubank Jr vs Wanik Awidjan
October 24 - Top Rank in Atlanta
Jamel Herring vs Shakur Stevenson - WBO super-featherweight title
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
November 20 - Top Rank in Las Vegas
Terence Crawford vs Shawn Porter - WBO welterweight title
December 11 - Top Rank in Las Vegas
Vasiliy Lomachenko vs Richard Commey
December 18 - Top Rank in Glasgow
Josh Taylor vs Jack Catterall - undisputed super-lightweight title