On Saturday, when the crowd at the O2 Arena will be roaring all around them, when they will be fighting for the undisputed middleweight championship of the world, when they can finally put to rest a rivalry that has festered for more than a decade, all that noise and fury can fall away.

Because at last they can box, and for Marshall when she steps through the ropes it's simple. "It's like home," she says.

That is because she has been working for this for so long. At the outset of their preparations her trainer Peter Fury warned her camp would be brutal. He was being perfectly honest.

• Boxing's fiercest feud - how the rivalry began
• Watch the original clash: Marshall vs Shields 2012
• When Marshall sparred Tommy Fury

Marshall has taken her training to a new level for this fight. "It was awful. It's more the duration of it, normally I do six to eight [weeks], I've done 12. Even the rounds of sparring. Normally I do two or three 10 rounders, I must have done six to eight. Everything's just been ramped up," she told Sky Sports.

Now, apart from light sessions on Thursday and Friday to loosen up, all that's left to do is fight.

"It is the fun part," Marshall said. "Everyone keeps saying, 'How are you feeling?' I'm just like fine. I think because Sky have let me headline my past two fights, it's kind of a bit of a taster. It's kind of getting used to it being about you and your opponent.

"For me it's just about savouring every moment. For when I come through this fight, I'm never going to get another fight, another build up like this ever. Ever. This is it."

They are not just fighting for all the belts, and they just fighting for the legacy of headlining a historic all-women's boxing bill on Saturday. They are fighting because it is personal.

"Her attitude. Everyone is coming here to watch the Brit knock the American out. This is what sold that fight," she said. "No one's coming to watch her. Everyone's coming to watch the Brit knock the gobby American out. That is what it is.

"I'm never going to get a fight like this, where there's a rivalry. Where it's a little bit more than business. It is a little bit more than business," she continued. "This is the biggest fight for me.

"There's me and Claressa. This is it. This is the one."

Marshall remains the only boxer to have beaten Shields, amateur or pro. It's a score the American is determined to settle.

It's irresistible to look into their shared past to try to discern what that might reveal about Saturday's fight. It was Marshall who beat Shields when they were young amateurs but it was Shields who went on to win Olympic medals and more professional titles.

They have only been in the ring once since. That was a spar before the 2016 Olympics. Some accounts suggest Shields had the best of it, others Marshall.

Marshall does not see it like that. "Genuinely to me, nothing happened," she laughed.

"My recollection is that we did four rounds and it was very cagey, neither of us gave anything away.

"I've said previously she threw a couple of punches a round and vice versa," she continued.

"It was a two-week training camp and we were the only two middleweights there and she didn't spar for 10 days. She'd hurt her back, she'd hurt her wrist, and it come to the last spar and she sparred me.

"She's a future opponent so why would I go in there and show here everything I've got, it just doesn't happen."

It's doubtful that either thought it would be such a long time until they actually fought again.

Marshall is convinced this contest will be worth the wait. She's not going to box like did in the amateurs either. Marshall will be looking to dominate. "Bully the bully" is how she described her approach.

But neither is this bout a simple case of Marshall being 'the puncher' and Shields being 'the boxer'.

"I do genuinely believe she can punch. I think all middleweight women can whack. It's just knowing how to punch properly," Marshall said.

"I think she's not a risk-taker, she's scared of taking risks. Whereas I'm different. I'll take a punch to knock you out.

"She likes not getting in trouble, which is fair enough, she's still winning and that's how I think she likes it.

"She likes to be in control."

Marshall is convinced she is better finisher and that will show that on Saturday, eventually.

"The way I keep looking at it is, I've trained for the best Claressa, I'm expecting a hard night," she said.

"To be honest if I don't get it, I'll be disappointed."

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.