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Holly Holm (in white) defeated Ronda Rousey in the biggest upset in UFC history. Getty

In easily the biggest upset in women's UFC, bantamweight champion Ronda Rousey was knocked out by relative unknown Holly Holm in the second round on Sunday in front of a record 56,214 at Etihad Stadium in Melbourne, Australia.

Holm, who previously was a boxing welterweight champion, delivered a left-footed kick to Rousey's neck that floored the champion. Holm then quickly seized on the opportunity to throw punches with Rousey on the mat, and referee Herb Dean quickly put an end to the bout with Rousey appearing defenseless and taking punishment.

Doctors attended to Rousey immediately after the fight. She was later able to stand for the official announcement of Holm as the winner.

Holm was a big underdog entering the bout. Bovada.lv listed Rousey odds at -2000, and Holm at +900. Rousey had been given -310 odds to win by submission. The odds of the fight lasting less than one round were listed at -210.

"I'm trying to take it all in. This is crazy," Holm said. "Getting in here, I just felt so much support. I thought, 'How can I not do this with all this support?' I had the best coaching, from standup to grappling to wrestling.

"I have to say, everything we worked on presented itself in the fight. Every grab she tried to get, on the cage. I have not spent this much time in the gym before any fight in my life."

Rousey, who previously claimed Holm had disrespected her, didn't touch gloves with Holm to start the fight. Rousey and Holm even had to be separated at the weigh in, as a scuffle nearly broke out.

But Rousey decided to box with Holm, which turned out to be a mistake considering Holm would win the opening round in what was essentially a fist fight. Holm landed 72 percent of her punches compared to Rousey, who landed 30 percent.

The knockout came after 59 seconds in the second round.

It was the quickest finish to a fight in Holm's career. The new undefeated champion has won seven of her 10 bouts by knockout.

honda rousey vs holly holm 15-11-2015by MALHACAOAOVIVO