De Fries began training in Brazilian jiu-jitsu at the age of 14, and also worked as a drugs counselor before becoming a professional fighter. Since the very beginning of his MMA career, De Fries has shown great submission skills, defeating the likes of Darren Towler and Stav Economou. He made his professional debut in 2009.
Ultimate Fighting Championship
After his win over Stav Economou, De Fries was scouted and has since signed a fight deal with the UFC. De Fries was expected to make his debut against Oli Thompson at UFC 138. However, on 17 October 2011, it was revealed that Thompson had to pull out of the bout due to injury and was replaced by Rob Broughton. De Fries won the fight via unanimous decision. In his second UFC fight, De Fries faced Stipe Miocic on 15 February 2012 at UFC on Fuel TV 1. He lost the fight via KO in the first round. De Fries faced former strongman Oli Thompson at UFC on Fox 4. He won the fight via submission in the second round. De Fries was expected to face Matt Mitrione on 29 December 2012 at UFC 155. However, Mitrione stepped up to fight Roy Nelson at and was replaced by Todd Duffee. De Fries lost via first round TKO due to punches. Mitrione vs. De Fries eventually took place on 6 April 2013 at UFC on Fuel TV 9. He lost the bout via knockout in the first round. Following the loss De Fries was released from the promotion.
Post-UFC
Following his release from the UFC, De Fries return to action on 31 December2013 in Japan as he faced Strikeforce veteran Brett Rogers at Inoki Bom-Ba-Ye 2013. He won the fight via rear naked choke submission in the first round. He then faced Judo specialist Satoshi Ishii on the IGF 1 card which took place on 5 April 2014. De Fries lost this fight via unanimous decision.
De Fries made his Bellator MMA debut on 15 December 2017 at Bellator 191 against Pride FC vet James Thompson. He won the fight via submission in the first round.
KSW
On 8 February 2018 it was announced that De Fries had signed a four-fight contract with Polish MMA promotion Konfrontacja Sztuk Walki. In the promotional debut De Fries faced Polish prospect Michał Andryszak for the vacant heavyweight championship at on 14 April 2018. De Fries won the fight via technical knockout in the first round and claimed the championship. On 6 October 2018 De Fries made his first title defense against Karol Bedorf at, retaining his championship via keylock submission in the second round. De Fries successfully defended his title against reigning KSW Light Heavyweight championTomasz Narkun at on 23 March 2019, winning the fight via unanimous decision. De Fries was initially expected to make his next title defense against Damian Grabowski at on September 14, 2019. However, Grabowski was forced to withdraw from the bout due to an injury, and was replaced by Luis Henrique. De Fries won the fight by split decision.