It came at the 2012 AIBA world championships in China in May 2012.

"I remember when I got there everyone was talking about this young American that had never been beat. She was the next superstar," Marshall recalls.

Marshall was preparing for the London Olympics while Shields was chasing qualification. It was only the second international tournament Shields had competed in and she was fresh off a shock win against three-time world champion Mary Spencer at the continental championships that summer.

The two women faced off in a fight that has gone down in women's boxing folklore. Marshall emerged the victor on points, 14-8. She says she won easily, Shields believes she edged it.

Marshall believes that defeat, the only loss of Shields' entire fighting career to date, "eats" away at her. Marshall won the world championships a few days later, Shields the London Olympics in August. This was the foundations of their rivalry.

"We can't erase the loss in the amateurs because I don't care about it," Shields says.

"I'm going to show everybody I was better than her then and I'm better than her now."

Coral Barry
BBC Sport combat sports editor

Savannah Marshall and Claressa Shields have known each other a long time.

Shields was only 17 when they first met, Marshall had just turned 21. They were both rising stars in the amateur game at the time, weeks away from the London 2012 Olympics.

Ten years on, they have become bitter rivals and two world champions at the peak of their powers.

When Shields, enters a room already occupied by Marshall, tensions rise. The pair are often kept apart, ushered into events at different times, meeting briefly for a face-off that usually ends in a shouting match.

It's a rivalry unlikely to be put to bed tonight.

Claressa Shields has held multiple world championships in three weight classes and the 27-year-old calls herself the greatest woman of all-time (GWOAT).

Meanwhile, Savannah Marshall has held the WBO middleweight title since 2020.

The 31-year-old Briton is the only boxer to have beaten Shields - when they met as amateurs in 2012 - and doesn't mind bringing it up every now and then... as Shields also likes to highlight often.

Savannah Marshall says she almost retired after some "broken promises" during her career.

Now the 31-year-old has the chance to become the first female undisputed champion from the UK.

She may have beaten Claressa Shields at amateur level but having gone out in the early rounds at the London 2012 and Rio 2016 Olympics, that would be some achievement.

Here's our Born to Brawl on the Hartlepool fighter.

Coral Barry
BBC Sport combat sports editor

Claressa Shields is the brash, outspoken American boxer who toes the line between villain and superhero, as she puts it.

Google her name and you will find images and videos of a woman full of fire, never backing down from a fight, wherever it presented itself.

Tonight, she faces long-time rival and WBO champion Savannah Marshall in a contest for the undisputed middleweight championship.

But Shields wasn't always the force of nature she is now.

She cites her tough upbringing - where she didn't have a bed of her own to sleep in until she returned from the Olympics as a gold medallist aged 17 - for making her the fighter she is.

"People like to say I'm unhinged but I would say I'm a pretty great person. And I think Flint had a lot to do that," she says.

Shields was raised in Flint, Michigan, which regularly tops the charts as one of the poorest cities in the US. "I definitely grew up in poverty," she adds.

Click here for Shields' full story.

Image source, BBC Sport

A month later than planned, what promises to be one of the fights of the year is finally here.

After being postponed following the death of Queen Elizabeth II, Claressa Shields' clash with her old nemesis Savannah Marshall for the undisputed middleweight crown is happening tonight.

That tops the first all-female boxing card held in the UK and it's the first time two female fighters have headlined at London's O2 Arena.

We'll be bringing you live updates on all the action later, plus mention former heavyweight champion Deontay Wilder's return to the ring against Robert Helenius in New York.