A multimillion dollar mega fight in women's MMA went up in flames when Amanda Nunes suffered a shock loss

• Amanda Nunes lost a fight she was expected to win Saturday at the UFC 269 event.
• The result threw a mega-fight worth millions of dollars into disarray.

A potential UFC mega-fight went down the drain Saturday night, and with it millions of dollars, when UFC fighter Amanda Nunes lost, scuppering a potential showdown with Kayla Harrison — a millionaire combat sports superstar from rival fight firm PFL.

Nunes entered the Octagon as the pound-for-pound No.1 women's MMA fighter on the planet, and was heavily-favored to defeat Julianna Pena, who was considered an underdog with little chance to compete, let alone triumph.

The 33-year-old Nunes had run a gauntlet of the toughest women in the UFC roster, taking out Ronda Rousey, Valentina Shevchenko, Cris Cyborg, and Holly Holm during her run as the fight firm's bantamweight queen.

Pena, meanwhile, had lost as many bouts as she had won in the four fights prior to UFC 269, which took place December 11 in Las Vegas.

Nunes looked authoritative in the opening round on the night, out-striking Pena and taking her to the ground, but Pena came back in the second, and boy, did she come back strong.

Pena stunned Nunes with stinging punch combinations and turned the heat up so high she landed 74 of her 115 strikes (64%), and succeeded with her one takedown.

When down, Pena locked in a rear-naked choke and Nunes tapped.

The UFC's own social media account called the result: "The biggest upset in UFC history!"

It was all over.

"Amanda's been such a great champion, and she's done so much for the sport, so for me to take down arguably the greatest of all time is something that's still sinking in right now," Pena told reporters after her win.

Considering Nunes' reign, there's talk of a rematch.

"We can do it next," Pena said. "I'm free next month, two months from now. Whenever they want to do it, I'm ready. If she wants to do a rematch, we can do a rematch."

A mega-fight worth millions of dollars is now in tatters

As a two-time gold medal-winning judoka at the Olympics, and a two-time PFL MMA champion, Kayla Harrison is one of the most decorated combatants in fight sports.

Harrison spent five years at PFL, built an impeccable pro MMA record of 12 wins (five knockouts, five submissions, and two decisions) while remaining unbeaten, and earned more than $2 million in prize money.

But she has now entered free agency, and though she could re-sign with PFL to try and win a third $1 million championship check, there have been rumors of discussions with UFC decision-makers.

A bout between Harrison — who once told Insider that comparisons to Rousey were unfair because she's actually way better — and Nunes appeared closer than ever.

It would have been a fight that would generate a considerable amount of money.

Harrison, watching Octagonside, was all-smiles before the fight began but was heard screaming "fuck!" when the match had finished and the Nunes mega-fight was thrown into disarray.

Reporters at a post-fight press conference asked the UFC boss Dana White for his thoughts on the issue.

"If Amanda Nunes had won tonight, the Kayla Harrison fight would be one of the biggest fights that you would have ever seen," he said.

"These are those things, man. For like a Kayla Harrison, the Amanda Nunes fight was huge. I mean, if they would have faced off, it would have been a massive, you know, mega-million-dollar fight."

It is currently unclear if the UFC will sign Harrison regardless, or if she'll return to the PFL for one more season.