He would notch up three more wins in the Dutch circuit, defeating Michael Lorenz, Thomas Bridgewater and Nico Pereira Horta.
[5][6] Mannaart participated in the 2017 K-1 Heavyweight Grand Prix, being scheduled to fight Masahiro Iwashita in the quarterfinals.
[8] Four months later, Mannaart was scheduled to fight Antonio Plazibat for the K-1 Heavyweight title at K-1: K'FESTA.1.
[12] Mannaart was scheduled to defend his K-1 title against Chris Bradford, at K-1 World GP 2019 Yokohamatsuri.
[15] Maslobojev won the fight by a first round TKO, with Mannaart suffering a broken jaw during the bout.