KSI was elated backstage when he saw his little brother Deji win his first ever boxing victory.
Fousey, a YouTuber himself, entered the 02 fight having lost his three previous fights.
Fousey defeated by Deji in brutal fashion
The 25-year old won his light heavyweight bout
He put the slump to rest and saw off the American in brutal fashion.
Fousey had been dropped by Deji with a jab. Deji then ended round three with a rocking of his opponent with big lefts, prompting him to throw in the towel.
Backstage, KSI, headline act and big brother to KSI, was watching. He had just defeated Swarmz in his first fight.
He had tipped his brother, aged 29, to lose the fight before Deji's victory.
Backstage footage shows KSI looking tense as he anxiously watches the final stages.
Before he could utter "you've got it", he was overcome with joy at the end of the fight.
KSI screamed repeatedly "yes!" while jumping in joy and also smacking his gloved hands together.
He said that his brother was extremely proud of him and spoke about his success. It was one of the most memorable moments in my life.
He wanted to show off his skills. He took his time. He was able to comfortably go in and out, jabbing after jabbing.
"He did the cross over and over again, punching Fousey in his face.
"Deji broke it, and I'm so proud. I've told him many times. He had it in him, I told him.
"I told him that he was up there with the top boxers on YouTube and he proved it tonight. "I am so proud."
Deji said, "I have my best team." Daley is the greatest boxing coach, and I am proud to have him.
"I have Leon, who is the best PT in this game, and I have everyone showing their love." That brought me out."
After seeing his brother win, KSI was able to easily beat Luis Pineda in the third round via TKO.
KSI defeated Luis Pineda easily