After amassing an 11-3 record on the regional scene, including an eight-fight winning streak after his 3-3 start to his professional career, Moreno appeared on The Ultimate Fighter: Tournament of Champions. There, Moreno ended up on Joseph Benavidez’s team as the lowest-ranked seed and lost his first fight via submission to Alexandre Pantoja. Moreno had a chance to turn things around quickly, however, as he made his UFC debut against Louis Smolka while his TUF season aired. He picked up a submission win and a performance bonus, and the ball was rolling for Moreno. 

He fought four more times over the next year-and-a-half, beating Ryan Benoit and Dustin Ortiz but losing back-to-back fights to Sergio Pettis and Pantoja. Despite a solid 3-2 run in the Octagon, Moreno was cut as the flyweight division found itself in limbo. However, Moreno surged back in June 2019, winning the LFA flyweight title with a fourth-round TKO win over Maikel Pérez, and that was enough to get him back with the mixed martial arts leader. 

Although his return bout with Askar Askarov in Mexico City ended in a split draw, Moreno returned to the Octagon looking immensely improved and much more confident, particularly with his standup skills. He followed that performance with rather impressive decision wins over Kai Kara-France at UFC 245 and Jussier Formiga in Brazil to put him in a No. 1 contender bout against Brandon Royval at UFC 255. There, Moreno got Royval out of there in less than a round to distinguish himself in the title picture. Following Deiveson Figueiredo’s demolition of Alex Perez at the same event, UFC asked both men to come back three weeks later at UFC 256 in what would be the start of a legendary rivalry.