Groenhart began his kickboxing career fighting on small shows in the Netherlands. He made his SLAMM debut in 2006, losing both of his fights with the organization, including a decision loss to Leroy Kaestner at B-Klass level. In 2008, Groenhart was invited by the K-1 organization to take part in an eight-man tournament, which had originally been intended for heavyweights but was changed later for fighters around the 75 kg bracket. Groenhart won the competition in spectacular fashion, dispatching all three of his opponents by stoppage on the night to take the title in Milan. The performance at the K-1 tournament led to Groenhart being invited to take part in a fight within the It's Showtime organization – the Netherlands biggest kickboxing organization. He lost his debut to It's Showtime Reality winner Sem Braan but impressed enough to be invited back for further matches. Although an impressive fighter with a good knockout ratio, Groenhart has had up and down performances within the organization, winning fights against the likes of Yassin Boudrouz and Joep Beerenpoot by knockout, but losing to Vladimir Moravcik and being knocked out by Ali Cenik, fights he should have been expected to do better in. Groenhart has also fought and lost to top class fighters such as Cosmo Alexandre and Nieky Holzken – with the loss against Holzken after an extension round, being slightly unfortunate for some who felt Groenhart deserved more. Despite some inconsistent performances, Groenhart won the vacant E.M.T.A. European title at the start of 2010, defeating Amir Zeyada in an impressive showing. Groenhart ventured up to the 79 kg division to face Marc de Bonte at ' on October 6, 2012 in Brussels, Belgium and scored another come-from-behind victory as he knocked his opponent out with a knee strike in the second round. Groenhart went on to win the K-1 World MAX 2012 World Championship Tournament Final in Athens, Greece on December 15, 2012, stopping Yasuhiro Kido and Mike Zambidis before knocking out his stable-mate Artur Kyshenko in the final. Groenhart faced Robin van Roosmalen at ' in Milan, Italy on April 20, 2013, losing a unanimous decision after getting dropped in round one. Groenhart lost to Davit Kiria via unanimous decision at ' in Ontario, California, United States on September 28, 2013. He joined the Blackzilians camp in preparation for the fight. Groenhart was expected to fight Shemsi Beqiri on the ' undercard in Zagreb, Croatia on March 8, 2014 but Beqiri withdrew due to an injury was replaced by Teo Mikelić. Mikelić dropped him with a right cross early in round one but Groenhart climbed off the canvas and went to work, opening a cut on the Croatian that prompted a doctor's stoppage after the first round. Replacing his stablemate Artur Kyshenko who conceded that he would be unable to make the contracted weight of -71 kg/156 lb, Groenhart stepped in to face Dzhabar Askerov at Legend 3: Pour Homme in Milan, Italy on April 5, 2014. He was fortunate not to be counted by referee Atsushi Onari in the early goings after succumbing to knockdown courtesy of a left hook by Askerov, but he soon made quick work of the Russian, dropping him with a right hook before putting him away with a left hook inside the opening round.
