Hrgovic – wearing the red and white checks of his country – made his entrance first to a wall of boos, before the mood inside the O2 Arena flipped as Itauma emerged to a huge roar – taking his time and soaking up the reception.
The crowd had made its allegiances clear after a fiery build-up which included Itauma shoving Hrgovic, the Zagreb man slapping him back and plenty of verbal sparring.
Itauma came out with real intent, landing straight lefts from his southpaw stance. Hrgovic is known for his durability but struggled to keep pace with the Briton’s speed early on.
Early in the second, a right hook knocked Hrgovic off-balance and his glove touched the canvas. Oasis star Noel Gallagher and boxing legend Prince Naseem Hamed were among those watching as the crowd gasped at almost every Itauma punch, whether it landed or not.
But by the end of the third, Itauma – who had boxed just 26 professional rounds before his first world-title fight – was already breathing heavily.
There had also been questions over his chin, simply because it had never really been tested. Hrgovic began landing the occasional heavy shot but Itauma took them well and fired straight back.
A fight in the crowd momentarily took attention away from the action in the fifth before an Itauma uppercut in the closing seconds brought the focus back to the ring.
Itauma continued to fire combinations but Hrgovic was barely budging. A huge right landed on the bell in the sixth and another followed in the seventh, yet the Croatian simply walked through them.
The Briton was well ahead but, rather than box his way towards victory, he continued to chase the knockout. A flurry in the eighth backed Hrgovic on to the ropes and had him in trouble, but Itauma was grimacing between exchanges and looking increasingly spent.
By now the danger was clear. Hrgovic came out with renewed purpose in the ninth and landed a thudding right which sent Itauma staggering backwards, almost through the ropes.
The youngster somehow stayed upright but his legs had gone. Hrgovic continued to press and when Itauma slumped into his corner, exhausted and barely able to defend himself, the towel came in.