They've got millions of fans worldwide and tonight will go head-to-head in the boxing ring in "the biggest event in internet history".
KSI and Logan Paul are no boxers, but the grudge match between the two YouTube stars has as much hype as any recent showdown.
With a combined following of more than 37 million subscribers, they are two of the biggest internet stars in the world.
Prankster Logan Paul was the subject of a global backlash earlier this year after posting a video showing the body of a suicide victim in Japan. The 23-year-old American later apologised over the incident.
British star KSI, real name Olajide William Olatunji, built a YouTube following based on his gaming and music.
KSI, aged 25, beat fellow YouTuber Joe Weller in a boxing match earlier this year to claim the YouTube Boxing Championship, a belt which will be on the line when he meets Paul on Saturday.
More than 1.5 million people watched the KSI v Weller fight live and more than 35 million have viewed it on YouTube since.
The KSI v Logan Paul bout is expected to bring in even bigger numbers.
Around 20,000 fans will watch the fight live at the Manchester Arena, but millions more will tune in via pay-per-view.
The stream is available for £7.50, which includes the undercard and main event, either via YouTube or at www.ksivslogan.tv.
The first fight will be at around 5.30pm, with the main event taking place some time between 8pm and 9pm. Their brothers Jake Paul and Deji will fight on the undercard.
Since agreeing to the bout earlier this year, Paul and KSI have traded insults via videos on their respective YouTube channels.
Paul, a former state-level wrestler, is widely considered to be the favourite to win.
He weighed in slightly heavier at 86kg to KSI's 84.9kg.