[2] Since the very beginning of his MMA career, De Fries has shown great submission skills, defeating the likes of Darren Towler and Stav Economou.
After his win over Stav Economou, De Fries was scouted and signed a fight deal with the UFC.
[4] 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.
[15] Following his release from the UFC, De Fries returned to action on 31 December 2013 in Japan as he faced Strikeforce veteran Brett Rogers at Inoki Bom-Ba-Ye 2013.
[20] On 8 February 2018 it was announced that De Fries had signed a four-fight contract with Polish MMA promotion Konfrontacja Sztuk Walki.
[23] On 6 October 2018 De Fries made his first title defense against Karol Bedorf at KSW 45: The Return to Wembley, retaining his championship via keylock submission in the second round.
[26] De Fries was initially expected to make his next title defense against Damian Grabowski at KSW 50 on 14 September 2019.
[35] He defended his title and won the bout via TKO after Stošić tapped due to combination of exhaustion and damage in the fifth round.
[42] He won the bout and defended the belt after putting Bajor unconscious with a rear-naked choke at the end of the second round.
[43] De Fries faced Augusto Sakai at KSW 95 on June 7, 2024, winning by unanimous decision.
[44] De Fries faced Josh Barnett in a grappling match at XTB KSW Epic on February 24, 2024.