More than 10 years ago the two had boxed in the opening stages of the amateur World championships. It was not only the most painful loss of Shields' boxing career, it remains the only loss of her boxing career.

Just 17 years old, Shields longed to become an Olympian. This though was the only qualification tournament and, after losing early on, for that to happen the American needed the boxer that beat her to win the middleweight final.

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So Shields sat in the stands, watched Marshall and prayed.

"God, please give her the strength, give her the power and give her the speed to beat these girls. Take what's in me and give it to her and make her win," Shields thought.

"Because I needed the Olympics," she said. "Not just for boxing but for my life. It definitely changed my life."

Marshall did win. She became only the second English boxer ever to take the world amateur title. It was on the day of her 21st birthday.

Marshall remembered of Shields, "After I won the tournament, she came up to me and gave me a big massive hug and thanked me for winning the tournament because she qualified for the Olympic Games."

That brief pause in hostilities is meaningless now. For the last decade the rivalry between them has intensified and grown ever more bitter.

Marshall remains the only boxer to have beaten Shields and for the American that unanswered defeat, burning red on her record, is still raw.

They will fight once again, now as professionals, for all four of the major middleweight titles, at the O2 Arena in London on Saturday, live on Sky Sports.

"I've been undisputed twice, this will be the third time for me. I've already been undisputed at 160lbs before. It means a lot to be undisputed. But it means more to avenge my only loss," Shields said.

"I am the greatest woman of all time. I have proved that, but there's still just that one blemish and she's sitting here and I want to show her that, yeah, you beat me when I was 17. But I'm 27 now and I've grown a lot since I was 17. Mentally and physically, and I just want to keep proving I'm the best."

Shields accomplished an unprecedented array of feats afterwards. She became the first American boxer to become a two-time Olympic gold medallist before winning professional world titles across multiple weight classes.

But she didn't get over that loss for years. Maybe she still hasn't.

"I was so emotional and dramatic when I was 17. I had been crying the whole night because I was distraught and I was just upset that I had lost. I was an undefeated fighter, I was 26-0 and then I became 26-1, so I just remember crying and just trying to let it go," she recalled.

"I was just so mad because I had trained so hard."

Even going to a training session the next day, she dropped her kit bag and couldn't help bursting into tears again.

"I remember Tiara Brown and Queen Underwood [her team-mates, saying,] 'Champ, it's alright,' she recalled.

"'No, it's not!' I was just so mad.

"When we got home I sulked in that loss."

For years afterwards, when watching a section about that tournament in her documentary, she'd still cry. That only changed in 2016.

"It was inevitable for me to cry but after I won my second Olympics I watched it and I don't cry no more," she said.

"I'm not so angry about it and I know that it's just part of my story. I'm happy that story has got us here."

For Marshall herself that single win was momentous. Shields was the phenomenon ahead of that tournament. The teenager had beaten Mary Spencer, the three-time and then reigning world champion, the top boxer in their division. Shields was the hope of USA boxing, even at such a young age. Camera crews followed her. She featured in the New Yorker, Time magazine and major American media.

"I remember getting to that tournament and everybody was talking about this girl, this new American who is unbeaten," Marshall said. "Then the draw sheet come out and it was: 'Savannah you've got the winner of USA and India' and I remember thinking, 'Great, typical'.

"And then I remember thinking, 'I've worked this hard, who the hell are you? You've got two arms, two legs, that's it.'"

Lee Pullen, one of the GB coaches, was in Marshall's corner for that bout with Shields. "Warming her up, she was positive. She was calm, she was positive, she had that mental attitude going in there," he said. "It didn't phase or spook Sav at all, she handled it terrifically.

"She used her height, her reach and her boxing brain and ability and experience and beat her, and she did beat her as well. They're saying it was close but it wasn't close. There was only one winner.

"It was competitive, really competitive. It wasn't one-sided, it wasn't like Sav was miles too good for her. It was a competitive bout that Sav was edging all the way through, in my opinion."

Clearly the taller, Marshall had used that reach and range in the contest. As Shields tried to close her down, the Briton moved off, landing her jab and shifting from side to side.

They were level after the first round but in the second Marshall opened up a lead. She never let it slip.

Shields went after her in the third, she knew she had to try to force momentum back her way. Marshall was steady behind her jab, darting it in. Shields reeled off a good combination but Marshall let her right streak across when they were up close.

"I was on her like a pitbull on a goddamn rabbit. I chased her all over the ring. I chased her, she's sweating," Shields insisted.

"Holding, holding, holding," the American added. "Running, running, running.

"But you beat me up? I put everything on the line to win that fight. The only thing that wasn't in my favour was the judges that was it."

Marshall had a four-point lead going into the last round. Shields swung for her but the Englishwoman timed strikes with her lead left hook. Marshall might having been giving ground but she was landing shots. When she took punches on her arms, Marshall let a countering right cross go.

Her continual movement, shifting under shots and moving round Shields, prevented the American from pinning her down.

Shields clumped a right in on the bell but to no avail. Marshall had won 14-8.

With Marshall's hand raised, Shields turned away, for once and for the first time defeated. The tears would come later.

Marshall was more matter of fact in her recollections of the contest. "It was just another fight to me. There's nothing really that stands out about that fight," she insisted.

"It just so happens I beat this girl in 2012 on my way to becoming world champion.

"Obviously this was her first ever loss," Marshall added. "10 years on she's still whinging."

That result lit a fuse in Shields. "It happened when I was 17. All it did was make me work harder. That's what losses are supposed to do. It doesn't define you," she said.

The hurt, Shields promises, is over. "Losses don't define who you are. It's about how you bounce back and I bounced back pretty dang good," she declared.

"I proved not only am I great once but I'm great at the world championships 2014, I won gold there. She got fourth place, bronze, 2016. I won gold at the world championships again and then I won the Olympics again and she was there to witness all those moments and she had to sit there."

They would never box again as amateurs. Marshall was underperforming, most cruelly when, in 2012, with huge expectation on her at the London Olympics she lost her first bout.

"2012 was hell for me. It was awful," the Briton said. "I had people messaging me saying I've just put a grand on you to win gold.

"Going into that tournament I was ranked number one in the world, I was the favourite to win gold, I only had to win one fight to win a medal and I couldn't do it. It was just this overwhelming feeling of anxiety, pressure."

She lost then just before she would have boxed Shields. That happened to her too at the 2016 and 2014 Worlds.

At the Rio Olympics Marshall boxed well against Nouchka Fontijn, but was desperately unlucky for that decision to go against her and to leave that tournament without a medal.

Shields naturally doesn't see it that way. "As the years went on I beat so many girls that beat her. It showed that if she can't beat them, what is she going to do with me?" the American said of Marshall.

"I knocked them down like bowling pins and was always at the top. Strike! If she beat me up so bad in 2012 and was just unlucky in the 2012 Olympics, unlucky in 2016, unlucky in the world championships, both of those years and now she's unlucky in the pros, but I just suck, if that's the story we're rolling with, let's roll with it."

They never met in competition again. But they did share a ring once more. They sparred in America ahead of the 2016 Olympics.

The two boxers see that spar in different ways. "She couldn't do nothing with me and that was 2016, prior to the Olympics. That's why we see I won gold and she won? Nothing. You got served six rounds," Shields told Marshall. "Your face was orange as a pumpkin."

Marshall scoffed, "You've never made no one's face look like a pumpkin."

Pullen was there for the spar as well though. "Sav done really against her, really well," he recalled. "Sav just went straight at her and straight to work.

"The spar was good, I thought she'd take her time and have a little look but she didn't. She flew out the corner like a two bob rocket. It was good, it was a good competitive spar.

"I was really pleased and proud of her at the time, thinking she wasn't fazed at all."

"I've got nothing but respect for Shields to be honest," Pullen added. "I think Shields has gone on to do great things. She's won two Olympic gold medals and numerous other tournaments. She's set the world alight as a pro.

"It's a real good fight, well matched, may the best woman win. But for me it's Sav."

What might gall Shields most of all, is that, despite all of her accomplishments, Marshall could just be the boxer who has her number.

The only way to solve that riddle is to fight again.

"Also too it matters on who has lived in 2012 and who hasn't. She's lived there and she stays there. I've moved on," insisted Shields, the holder of the unified WBC, WBA and IBF middleweight titles.

"That was the biggest moment of her career and what some people do in this lifetime is when, it's the best thing that happens to them, they want to live there. But you can't live in one day. You have to continue to go and go and go. She got stuck in 2012.

"Going into the world championships I was the favourite. The fact that she beat me, it made her the favourite to win the Olympics. But once again I bounced back and won the Olympics three months later. She's lived there though. Every time she fights, she brings up Claressa Shields, Claressa Shields, Claressa Shields.

"She could never do it. She failed. And I won twice," Shields continued. "She doesn't know who I am as a fighter and she won't be able to figure it out in 20 minutes when we get inside the ring."

Marshall has gone on to win the WBO championship belt. Neither can rewind the clock but for the Briton, if she were to beat Shields and become undisputed, it would bring closure to all the years of frustration she's had in the sport amateur and pro.

"Now we're back on this path to cross again," Marshall said. "It's personality clashes. Because she's come up against girls in the past that are louder than me and she hasn't slated them like me. I think it's because she can't get under my skin and I think that annoys her.

"Because I'm English and she's American, it's because she's loud and I'm quiet, this is what has built it up to this massive fight.

"If you want to get into the whole nitty gritty of fate and all this, how that one fight became this massive grudge match."

The biggest fight in the history of women's boxing - Claressa Shields vs Savannah Marshall - is live on Sky Sports on Saturday September 10. Be part of history and buy tickets for the London showdown here.