The 7,500-capacity Copper Box was some way short of full, with the upper tier closed and empty seats dotted around the arena.
But Fisher’s “Bosh Army” did their best to fill the gaps with noise, giving the “Romford Bull” a raucous reception.
Alongside his social-media star father “Big John”, Fisher has built a loyal following through an unusually unfiltered glimpse into family life, training camps and the pair’s well-documented enthusiasm for Chinese takeaways.
Fisher started with intent, swinging heavy hooks and reddening Pirotton’s eye in the opening round.
The little-known Belgian was unfazed by the home support or Fisher’s pressure, but his eye was cut by the second.
“I can see, don’t stop the fight,” Pirotton told the referee before a doctor examined him in the fourth.
Allowed to continue, he came out firing, but Fisher responded with a flurry to head and body.
Blood stained Pirotton’s white shorts, yet he rallied in the fifth – his best round – as Fisher began to look tired and static.
Among those in attendance were British fighters Derek Chisora, Conor Benn and Caroline Dubois, alongside WWE stars AJ Styles and LA Knight.
WWE is owned by American sports and entertainment group TKO, which is also behind Zuffa Boxing.
The doctor took another look at Pirotton before the seventh but allowed the bout to continue.
Neither man was offering much by the latter stages until a short right hand from Fisher staggered Pirotton in the 10th.
For a moment, a 13th stoppage victory appeared within reach, but Fisher could not find the finishing burst as Pirotton slipped to his third loss in 15 fights.