The PFL is a combat organization structured in a similar way to the NFL or NBA as athletes compete for points in a regular season so that they can contest a playoffs and, should they continue winning, a lucrative tournament final.
For Invicta, Harrison weighed-in at featherweight rather than lightweight for a standalone fight against Courtney King.
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It was a bout that was a new experience for Harrison as she competed behind-closed-doors in a former church.
It was such a bloody affair that Harrison couldn't help but grimace when she walked away with the brutal win.
"It was just really bloody," Harrison told Insider. "I'm not naturally a psychopath or sociopath outside of the cage - I had a bit of remorse seeing all that blood."
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"Having to keep hitting her? I wasn't at war with myself because it's my job and I'm in the cage to do work, but I was definitely, like: 'Maybe we should stop this.'
"Then I got up and it was in my mouth, my hair, my fingernails. I took a shower, the shower was blood red, and I was like: 'This is what fighting is like, Kayla, so you're going to have to get used to it because this is going to happen again.'"
'This is what fighting is like, Kayla'
Fighting isn't just about stamping your authority on a match, blood and guts be damned. There's also the leveling-up in training camps, waiting for the next call to fight, and the politics behind the scenes.
There was plenty of waiting for Harrison in 2020. The PFL postponed its entire season amid the coronavirus pandemic, citing a desire to preserve the integrity of its league format.
The PFL gave fighters a monthly stipend of $1,000 to compensate, but it left athletes like Harrison frustrated as they did not have fight dates, and they felt like they were missing out on a prime year of their careers.
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So the opportunity to compete outside of the PFL, for Invicta, was huge, Harrison said. "I was very adamant that I compete in 2020," she told us.
"I think that was mentally one of the toughest times in my career because I wasn't injured, or sidelined by choice. It was something I couldn't control.
"When you're in the prime of your career and being told: 'Hey, you can't do this anymore.' That was beyond frustrating.
"I'm super thankful to Invicta and grateful for the PFL it worked out because nothing emulates a fight. You can't get that in sparring. So, to try a new weight class, throw elbows - that was big for me."
Between the last time Harrison competed for PFL, at the end of 2019, to her next match for the league Thursday on ESPN, Harrison has been working on her fundamentals as a fighter.
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As a two-time Olympic gold medal-winner in Judo, Harrison had great pedigree transitioning into mixed martial arts.
Rousey, she said, never won gold. Harrison won it twice.
Since moving into MMA, Harrison has been working on techniques which allow her to position herself to use her biggest strength: her grappling.
Harrison has been honing her skills at American Top Team in Florida, one of MMA's most-renowned gyms, home to celebrated athletes like Dustin Poirier, Amanda Nunes, and Adriano Moraes.
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"I'm blown away by how lucky I am to be able to train with the best team, the best fighters in the world, and the best coaches," Harrison said of her experiences at ATT under the tutelage of head coach Mike Brown.
"There's not a day that goes by where I'm not picking up something whether it's a technique, a specific move.
"My coaches are really good at developing a game plan," she said. "So many coaches struggle with, like, if they're a striking coach, they want you to be a striker. That's not really where I'm at with my team.
"I'm not going to throw spinning back kicks. I'm going to learn how to effectively get to my grappling and use techniques to do that.
"That's been pivotal to the success in my career as nobody's trying to make me be something I'm not," she said.
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Harrison says she remains the hunter, despite winning the PFL title
One of the greatest changes from the last PFL season Harrison competed in, to this, is that she now enters Thursday's fight against Mariana Morais as the champion.
From the outside looking in, it may appear that she's gone from being the hunter to the hunted, but this is not a mentality Harrison has.
"I've always had a target on my back, even since I moved to MMA," Harrison said. "Because of my judo career, Ronda [comparisons], and other things.
"I'm always the hunter, chasing greatness, and never satisfied with what I've got. This is what drove me to MMA.
"The night I won the Olympics, I just accomplished my life's dream, my goal, for the second time. But I woke up in the middle of the night with a panic attack, thinking: 'What now?' Find something to do. I'm always the hunter."
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Harrison said the 2021 field of athletes in the women's lightweight bracket is more competitive than it was in 2019, and says she's interested in seeing how everyone has developed during the pandemic.
On Thursday, she'll see first hand in her co-headlining bout against Morais.
PFL 3 begins Thursday at 7 p.m. ET on ESPN.
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