There's something eating away at Claressa Shields.
The self-proclaimed GWOAT — Greatest Woman Of All Time — is as brash and confident as her nickname suggests.
The thing is, she has the results to back it up.
The 27-year-old American has two Olympic golds and two world championship gold medals from her amateur career — and is a three-weight world champion with a 12-0 (2KOs) record in the professional ranks.
In fact, Shields is the only boxer in history, female or male, to hold all four major world titles at once — the WBA, WBC, IBF and WBO — across two weight classes, middleweight and light middleweight.
Incredibly, she won her first professional world titles as unified super middleweight champion.
It is a boxing career that's almost flawless. Almost perfect.
But there is just one blemish.
During her storied, title-laden amateur career, Shields suffered a single defeat in 65 fights.
That came a decade ago at the AIBA world championships in China at the hands of Englishwoman Savannah Marshall (12-0, 10KOs).
On Sunday (AEST) at London's O2 Arena — after a one month delay enforced by the death of Queen Elizabeth II — Shields is looking to avenge that defeat and attempt to reclaim the undisputed middleweight title.
It's being described as the biggest fight in women's boxing history.
Bad blood
As women's boxing developed in recent years, so too have the narratives developing around the fights and fighters.
This contest is a perfect example, with genuine animosity between the fighters that has possibly stewed over the past decade and really hit boiling point in the last year or so.
"Everyone is coming here to watch the Brit knock the American out. This is what sold that fight," Marshall told Sky Sports.
"No one's coming to watch her.
"Everyone's coming to watch the Brit knock the gobby American out."
It might be the case that too much is being made of a single amateur contest when Shields was 17 years old — which, now they are both professional, is pretty irrelevant.
It's especially spurious given that although Marshall lost that fight, she went on to dominate the amateur ranks, winning gold at consecutive subsequent world championships and Olympics, including in London.
Winning gold in 2012 at Marshall's home Games (the Englishwoman lost in the quarterfinals) led the American to tell Marshall that she "let her whole country down". Ouch.
Marshall has countered by highlighting Shield's MMA defeat last year to Abigail Montes — further proof, she says, that "the GWOAT is beatable".
Nevertheless, that shared history has sparked some serious bad blood between the two fighters.
After Shields beat Ema Kozin to retain her IBF, WBA and WBC middleweight titles in February, the pair traded insults ringside.
During the fight, Marshall pretended to fall asleep, saying afterwards, "people were walking out … It was embarrassing."
Shields responded by telling Marshall: "I'm chasing you."
The American was ringside when Marshall knocked out Femke Hermans in April to defend her WBO strap.
After inflicting a brutal third-round knockout of the Belgian with a thunderous left hook, Marshall instantly pointed outside the ring to where Shields was sitting, making it clear that she is next, with the pair again squaring up on the ring apron.
'I don't think that women's boxing will ever disappear again'
You could be excused for thinking that we've already had the biggest fight in women's boxing history earlier this year.
In May, Ireland's Katie Taylor — who plenty would consider the actual GOAT of women's boxing — beat Amanda Serrano of Puerto Rico at a sold-out Madison Square Garden.
That fight was the second-fastest to sell out boxing's most storied venue, highlighting that women's boxing can and will sell.
It helps that due to the relative lack of depth in women's boxing, the top names are forced to fight each other more regularly — that and the need to schedule higher-profile bouts to attract any interest amongst the plethora of men's bouts.
That's in stark contrast to the issues plaguing the men's scene over the past week or so, with the cancellation of the highly anticipated catchweight contest between Conor Benn and Chris Eubank Jr due to the former's positive drug test, and the latest breakdown of talks of the increasingly elusive heavyweight blockbuster between Tyson Fury and Anthony Joshua.
Women's boxing could and perhaps should provide a genuine tonic.
It's a credit to the development of the sport that following so closely on the heels of that historic night in New York, there's another card that elevates the sport even further.
In the co-main event in London, Mikaela Mayer (17-0, 5KOs) takes on Alycia Baumgardner (11-1, 7KOs) in a super featherweight unification bout — a fight with just as much simmering animosity between the fighters as the headline contest.
Mayer, who competed at the Rio Games on the same USA team as Shields, told the UK's Sky Sports that women's boxing had never been in a better place.
"I don't think that women's boxing will ever disappear again," she said.
"I say again because I feel like it did right after that period with Laila Ali and Mia St John and Anne Wolfe.
"There was a whole decade where women's boxing wasn't even a thing.
"[Now] we're back for good, I know that."
With more women making the jump from the amateur ranks into the professional game — including Aussies like Skye Nicolson (4-0) — Mayer may well be right.
"The talent pool is too deep," she said.
"There are some young girls coming up that are just going to blow everyone out of the water."
