LAS VEGAS -- Decorated kickboxer Gokhan Saki's crossover into mixed martial arts just faced a thundering setback.

Saki (1-2), a Dutch striker of Turkish descent, was unable to follow the smashing success of his UFC debut last September as one punch from Khalil Rountree Jr. led the way to a first-round knockout loss at UFC 226 inside T-Mobile Arena.

The underdog Rountree (7-2, 1 NC) found success on his feet early with counter right hands from the southpaw stance. But it was a 1-2 combination, with a pawing jab following by a straight left hand, that floored Saki before hammer punches from Rountree forced the stoppage at 1:36 of Round 1.

"It's very satisfying," Rountree said. "You know what? Yes, he's a kickboxer and all that stuff but when you come in this cage, it doesn't take just one punch to lay someone out. So here I am! I'm here!"

The 34-year-old Saki, a former K-1 World Grand Prix and Glory light heavyweight champion in kickboxing, lost his pro MMA debut in 2004 by knockout. But he made the full-time move to UFC in 2017 and turned heads but violently finishing Henrique da Silva with one punch in Japan last fall.

Rountree, 28, improved to 3-2 in UFC with all three victories coming by knockout.

"I didn't really know what to expect tonight. I think the big difference for me was not trying to. I focused on staying calm, being ready for anything. I was ready for low kicks, high kicks, clinches, getting flashed," Rountree said. "I just wanted to keep my cool and stay ready. From the moment this fight got announced, I've taken so much crap online, which was kind of shocking. It didn't motivate me - it just made this fight very personal for me. I wanted to go out there and fight my best fight, be the best me. I wanted to show the world what I was all about. And I did... I really want that bonus. I think I deserve it. 2 knockouts in a row. The long layoff when the fight got rescheduled. To go out there and put on a show, finish him in the first round. I really want the bonus.

"Now, it's time to focus on more training. I love being a student of the game. I love going to the gym and learning. I want to continue to improve every aspect of my game. I can't wait to be back in the Octagon."

For full results from UFC 226, check out our results page.