Opacic turned pro in 2016 and obtained some much-needed experience by joining the 2016 K-1 World Grand Prix -95 kg Championship Tournament. He then took his act overseas, competing at Kunlun Fight in China and an Enfusion tournament in Abu Dhabi.After years of building his reputation in the kickboxing world, Opacic finally joined ONE Championship and made his promotional debut at ONE: BIG BANG II in 2020. The card’s name turned out to be a premonition of things to come, as the Serbian striking ace knocked out Errol Zimmerman with a spectacular spinning heel kick. Opacic followed that up with three more highlight-reel knockouts, including one over respected heavyweight contender Francesko Xhaja in January 2022.