But the American began to tire in the third and KSI went headhunting, regularly finding a home for his overhand right in a shock turnaround. He continued to hold the upper hand in the fourth and looked to have levelled the fight at two rounds each.
The fifth was neck and neck as Paul responded, firing off a flurry of punches which rocked KSI and did enough to convince two of the judges he won the round.
KSI kept throwing bombs in the sixth and final round and definitely finished stronger as Paul finished the fight bleeding from his nose.
Both men agreed to a rematch. "I'll do it again, baby," KSI said. "Hell yeah, let's go."
"I feel like I won the fight but at the end of the day that's not what happened," Paul said.
"I'm going to fight KSI again."
KSI, real name Olajide Olatunjim, is from Watford and was trained by Floyd Mayweather Snr for the first of a scheduled two fights.
Paul is perhaps best known for posting a video of himself and a group of friends laughing at a dead body in Japan's Aokigahara forest — a place notorious for suicides.
Both men entered the ring wearing masks.
How did the brothers go?
Logan Paul's younger brother Jake defeated KSI's brother Deji in the chief undercard bout.
Jake Paul poured on the pressure in the six-rounder and was doing enough damage in the fifth to force Deji's team to throw in the towel.
"I feel amazing man, I worked so hard for this," Jake Paul said. "Deji fought a great fight, I have respect for him, but I hope he learned from some of the things he said. He talked s*** about my family, which wasn't cool, so I hope he learned from that."
The 21-year-old called out pop star Chris Brown after his win. "I know you were supposed to fight Soulja Boy, but I think it's time to fight someone your own size," Jake Paul said.
Deji wanted a rematch despite being overpowered. "I know I let a lot of people down," Deji said. "But I want you to know, this is not the end of me ... he surprised me, he hit hard."
Who else was on the undercard?
A host of other online stars competed on the undercard.
JMX defeated Coach Richard
MOMO defeated RossiHD
AnEsonGib defeated Jay Swingler
FaZe Sensei defeated overtflow
Rackaracka defeated Scarce
Call Me Ham defeated Jrizzy Jeremy