Tim Kennedy, a former Special Forces sniper and current Texas Army National Guard soldier, is leaving the mixed martial arts world after two decades.

Kennedy, who boasts an 18-6 record since 2001, announced his retirement via his Facebook page on Tuesday.

The 37-year-old middleweight fighter recently lost to Kelvin Gastelum at UFC 206 in Toronto in December — it was Kennedy's first fight in two years.

"A lot of my coaches, friends and fans immediately tried to build me up again. ... I appreciated their comments and I don't think they are wrong. I know I am still a good fighter. I know I was away a while," he wrote in the post. "But they didn't feel what I felt, and that's being 37."

Kennedy wrote that he felt like he was in "slow motion" during the match.

"I felt tired for the first time ever in a fight. I'm the guy that once graduated Ranger School — a place that starves you and denies you sleep for over two months — and took a fight six days later in the IFL and won. I'm the guy that is always in shape. And I was for this fight. I worked harder than I ever have before for this fight. But I wasn't me anymore," he said.

Kennedy wrote that as far as losses go, he was happy to lose to a fighter such as Gastelum. 

"[He is] a really respectful and hard-working young fighter who went out and did all the things I consider myself good at, but did them better," Kennedy wrote.

Kennedy thanked his family, friends, coaches, and even his opponents, for helping him throughout his MMA career. He also gave a shout-out to the military.

"I want to thank the military community for their support. I'll never be able to explain how much you motivated me and how much I always tried to make you proud. ... I will keep fighting for you all until the day I die," he wrote.

Kennedy gave a mysterious hint to what the future holds for him, thanking his wife and apologizing for having "already moved onto another super dangerous thing to fill" his time.

"So with that, to all of you fighters out there, I am not going anywhere," he wrote. "I love fighting and will always have the heart of a fighter. I am committed to growing our sport and taking care of those who are a part of it."

Charlsy Panzino covers the Guard and Reserve, training, technology, operations and features for Army Times and Air Force Times. Email her at cpanzino@militarytimes.com .