Anything they have done in the build-up to the fight has generated massive numbers on their social media platforms. His tilt with Logan Paul has opened up an even wider audience to his antics, and a bigger venue to measure up to those expectations.
All to jump on board the hype and publicity train.
Paul took a strong lead in the first two rounds, but KSI proved he had stamina and managed to fight his way back as their battle went on.
They did. If anyone was in any doubt about YouTube's place in society after 1.8 million watched February's bout for free, perhaps they'll pay attention now viewers have spent more than $8 million - plus ticket and merchandise sales - on small-screen superstars.
After six rounds, the bout was ruled a majority draw.
And with that many eyeballs on the fight, it brought in a ton of money.
However, The Verge notes that many watched pirated streams of the fight on Twitch or Periscope.
KSI appeared to dominate from thereon, particularly in the final round where he seemed to connect with a number of his punches.
"I guess we have to have a rematch", KSI said after fellow YouTuber True Geordie explained to him what a majority draw actually meant.
With Manchester Arena also seating 21,000 for boxing matches, and the average ticket costing around £100, that could give KSI another £2.1 million, bringing his possible weekend total to almost £8 million.
Paul's brother Jake defeated KSI's younger sibling Deji via TKO on the undercard and later called out singer-songwriter Chris Brown.