Dijak reported to the WWE Performance Center on August 21, 2017. His signing was announced by WWE on September 5.
He made his debut for WWE's developmental branch, NXT, on September 23. His television debut, under the name Chris Dijak, came on May 30, 2018, in a loss against Ricochet. In July 2018, WWE revealed Dijak's new ring name as Dominik Dijakovic. Promos started to appear on the December 5, 2018 episode of NXT, promoting the debut of Dominik Dijakovic. Dijakovic made his debut on the December 19 episode of NXT, defeating Aaron Mackey.
During WWE Worlds Collide at WrestleMania Axxess, Dijakovic lost to Harper. Following WrestleMania 35, he was set to feud with NXT North American Champion Velveteen Dream over the championship. However, in April 2019 he injured his knee, resulting in a torn meniscus that would require surgery. He would be out of action until late July, when he returned at an NXT live event. On the August 28 episode of NXT, Dijakovic defeated Keith Lee in a highly competitive match.
On November 13, 2019 episode of NXT, Dijakovic joined Team Ciampa for the annual WarGames match with Tommaso Ciampa, Matt Riddle and Keith Lee against The Undisputed Era at NXT TakeOver: WarGames, effectively turning face. Riddle proceeded to leave the team to pursue his rivalry with Finn Bálor. At the event, Dijakovic and Team Ciampa defeated The Undisputed Era, with Riddle's place being taken by Kevin Owens.
On January 29, 2020, Dijakovic defeated Damian Priest to earn a number one contendership spot against NXT North American Champion Keith Lee at NXT TakeOver: Portland on February 16, 2020. Dijakovic went on to lose to Lee after his back gave out in an attempt for his finishing move, the Feast Your Eyes. After the match, Lee and Dijakovic shook hands as a sign of respect, slowly transitioning Dijakovic into a fan favorite. After Lee's NXT Championship win against Adam Cole at NXT: The Great American Bash, Dijakovic was challenged by Lee once again in a winner takes all match on the following NXT show, with Dijakovic unsuccessfully capturing the titles. He then had a backstage encounter with Karrion Kross, setting up a match between the two for the following week's NXT, which Dijakovic would go on to lose.
On the September 21, 2020 episode of Raw, Dijakovic was officially revealed as one of the main members of the recently created faction known as Retribution, under the new ring name T-Bar. At Fastlane 2021, T-Bar, Mace, Slapjack and Reckoning seemingly turned face after double-crossing leader Mustafa Ali.
During the April 12, 2021 Raw, T-Bar showed signs of joining The Hurt Business after attacking Drew McIntyre, who defeated Braun Strowman and Randy Orton for a match against Bobby Lashley at WrestleMania: Backlash. The following week on Raw , T-Bar was finally unmasked along with Mace in a tag team match against McIntyre and Braun Strowman, in which T-Bar and Mace won by disqualification. Later in a interview, they confirmed the start of their tag team run. After a few weeks of absence from television, T-Bar and Mace were designated as lumberjacks in a lumberjack match between John Morrison and Damian Priest on the May 17 episode of Raw.
As part of the 2021 Draft, Mace was drafted to the SmackDown brand while T-Bar remained on the Raw brand, thus disbanding the team. On the October 25 episode of Raw, T-Bar got himself disqualified in a match against WWE United States Champion Damian Priest after throwing an office chair at the champion. For the remainder of 2021 and the majority of 2022, T-Bar would compete almost solely on Main Event.
On the October 25 episode of NXT, a vignette was shown of T-Bar's Retribution mask being tossed into a fire, teasing his return to the NXT brand. On the November 22 episode, he returned to the brand under the name Dijak, attacking Wes Lee after Wes had defended his NXT North American Championship against Carmelo Hayes. At NXT Vengeance Day, Dijak lost his North American Championship match due to interference from Tony D'Angelo and Channing "Stacks" Lorenzo. On March 7 at Roadblock, Dijak was defeated by D'Angelo in a Jailhouse Street Fight. In April, Dijak began a feud with Ilja Dragunov. At Spring Breakin', Dijak attacked Dragunov backstage. At Battleground on May 28, Dijak was defeated by Dragunov in a Last Man Standing match, ending the feud.
On the November 7 episode of NXT, Dijak defeated Tyler Bate to qualify for the Iron Survivor Challenge at NXT Deadline, which was won by Trick Williams. On the January 15 episode of NXT, Dijak faced Trey Bearhill, with Joe Gacy serving as commentator. During the match, Gacy headbutted Dijak. After Dijak won the match against Bearhill, Gacy attacked Dijak from behind and both men started brawling. The following week, a match between Dijak and Gacy was scheduled, but the match didn't happen due to Gacy attacking Dijak before the bell rang. Both men continued fighting until they were pulled apart by referees and security guards. On February 4, 2024, at Vengeance Day, Dijak defeated Joe Gacy in a no disqualification match. On the February 13 episode of NXT, Gacy faced Carmelo Hayes in a losing effort. After the match, Dijak attacked Gacy with a nightstick and then locked him in a straightjacket and left. The following week, Dijak locked Gacy in an asylum cell. On the February 27 episode of NXT, Dijak defeated Luca Crusifino. At NXT Roadblock on March 5, Dijak defeated Gacy once again, this time in an Asylum match. At NXT Stand and Deliver on April 6, Dijak failed to win the NXT North American Championship from Oba Femi in a triple threat match which also included Josh Briggs.
On night two of the 2024 WWE Draft on April 29, Dijak was drafted to the Raw brand in the supplemental draft, marking his return to the main roster after two years. On June 28, Dijak left WWE after only appearing on Main Event and WWE Speed.