KSI or Logan Paul?
YouTube fans around the world have chosen whose side they are on, and will be watching when the pair of social media heavyweights step into the ring at Manchester Arena.
Both are wildly popular, both are massively divisive, both have highly controversial pasts.
Who are KSI and Logan Paul?
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KSI is the brash 25-year-old British gamer, comedy vlogger and rapper whose videos have racked up 4.4 billion views despite heavy criticism for lewd comments towards women in some of his clips.
Logan Paul is the 23-year-old US prankster (and former state wrestler) who made his name on Vine before moving to YouTube, where he's had 3.9 billion views, and who caused outrage earlier this year for showing the body of an apparent suicide victim in Japan.
Both have cultivated a frenzied online feud in order to hype up this bout, which will be streamed pay-per-view on YouTube.
Their brothers Jake Paul (4.9 billion views) and Deji (3.3 billion) will fight on the undercard.
Both seem to have largely shrugged off their controversies, but the fight will be seen by some as a way to prop up, and cash in on, their careers.
Asked by BBC News whether he's hoping to redeem himself, Logan says: "I don't think this is the redemption. I think it may mark a part of it, but by no means is this fight a redemption for my mistake that happened in January.
"But I think it is an opportunity to for me to have taken a step back from the internet and focus my efforts elsewhere, and it's an opportunity for me to show the world a new version of Logan Paul, not just silly internet vlog boy."
KSI, when asked whether he has regrets about his past behaviour, tells BBC News: "I definitely have regrets for some of the things I've said and done, but you know, I'm a human being and human beings make mistakes.
"Because it's me, it's showcased all over the internet, but that's just how it is."
What the fans are saying
Social media can be a combative place, so perhaps it's logical for social superstars to take their hostility a into the real world.
At one of KSI's training sessions in Manchester before the fight, fan Shaf Miah, 25, from London, says the white-collar amateur bout is "the next level of internet beef".
He says: "If you read comments [on YouTube], people want to physically hit someone if they don't like them or if they've done something annoying. And to see their favourite YouTuber do it for them, they're obviously going to pay money to watch it."
Michael Zepeda, 21, has come from LA for the fight. "They've both got a lot of subscribers, they both get a lot of views," he says. "Everybody wants to watch.
"I want to see Logan get his ass beat. He deserves it. He's done many messed up things for views. He crossed that line, definitely."
CJ Melia, 21, has travelled to Manchester from Dublin. "I cannot wait," he says. "I'm so excited for this. I've been watching KSI for about nine years. He's the reason I started my first YouTube channel eight years ago.
"I never thought something like this would happen. This is the biggest event YouTube has ever done."
What the pros are saying
Media captionTyson Fury has a few thoughts
How the bout came about
This all started when two other YouTubers - Joe Weller and Malfoy - went into the ring last year, and KSI said he would fight the winner.
KSI took on Weller at the Copper Box Arena in London in February, and won. That was streamed live on YouTube for free, with 1.8 million people watching live and 36 million more watching on the pair's official channels since.
Weller has five million subscribers to Logan Paul's 18 million. Saturday's fight will be much bigger - but this time, punters have to pay £7.50 to watch.
The biggest event in internet history?
The Duke and Duchess of Cambridge might have something to say about that. Their 2011 wedding holds the Guinness World Record for the most live streams for a single event, at 72 million - although that obviously wasn't a product of the online world, as this fight is.
YouTuber Jordan Antle, who goes by the name TheFearRaiser, says the fight's billing is not over the top. "This is a massive event and I would definitely say this is going to be one of the biggest internet events so far in history," he tells BBC News.
"Seeing two YouTubers who have a large fanbase fighting each other live in front of millions of people in unheard of. It's a very strange combination of two worlds and it's attracting a lot of attention because of how different it is."
KSI and the Paul brothers have certainly been doing their best to whip their fans into a frenzy.
Insult-filled videos and diss tracks have flown back and forth, and ill-tempered press conferences have taken place in Los Angeles and London.
And on Wednesday, Deji turned up unannounced at the Pauls' training gym in Manchester and ended up getting a slap from Jake.
Is it for real?
The antics have all been more WWE than WBO. But KSI's manager Liam Chivers insists it's "100% real".
He says: "People think these guys are not professional boxers, this is just a bit of fun, it's just an amateur fight.
"No - these two guys are on an absolute pedestal and they have egos, they have hugely passionate audiences.
"This is the modern-day mainstream of fandom, this is where the pressure is at, and these two guys have built their reputations and audiences over however many years, and it's all on the line.
"Plus the fact they're promotional geniuses and they've applied that and have attacked each other so hard that they are really very upset with each other. There's not one little bit of fake beef about it."
How many people will watch?
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KSI also has a music career - his debut album reached 13 in the UK chart in 2016
More than 15,000 tickets have been sold for Manchester Arena, but Chivers is reluctant to predict how many people will sign up to watch online.
"It's really hard to know - this is the first time this has ever been done," he says. "If you said a number of a couple of hundred thousand buys, I'd say that was not good but that would cover costs."
The fight will cost £7.50 to watch in the UK or $10 in the US - much less than a big professional pay-per-view fight, but much more than people are used to paying on YouTube.
"There's a YouTube culture of not buying and it's a whole educational process to see how many tune in," Chivers says.
Taking part in high-profile real-life contests is nothing new for YouTubers - although, with Zoella on The Great Comic Relief Bake-Off in 2015 and her brother Joe Sugg in this year's Strictly Come Dancing, other British examples have been more sedate.
So with their attention-grabbing aggression, KSI and Logan Paul have stumbled across an effective way to boost their profiles and bank balances, and other YouTubers may follow suit.
No matter who wins bragging rights in Manchester - and the victor will brag a lot - the pair have already agreed to a rematch in the US. This beef has only just begun.