5 biggest upsets in UFC history

Julianna Peña shocked the world at UFC 269 when she defeated Amanda Nunes to win the women's bantamweight championship. Peña submitted Nunes in the second round of their title fight on Saturday in Las Vegas, Nevada.

Heading into that bout, Nunes was a -1100 favorite and was riding a 12-fight win streak. Few anticipated Peña providing much of a challenge for 'The Lioness' but 'The Venezuelan Vixen' proved all doubters emphatically wrong.

This is not the first time fans have seen massive underdogs pull out the most unlikely of victories in spectacular fashion. These moments serve as a reminder of how unpredictable MMA can truly be.

Here are the five biggest upsets in UFC history:

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#5. Anderson Silva vs. Chris Weidman - UFC 162

Wiedman fought Anderson Silva in just his sixth UFC fight

For years, Anderson Silva was completely untouchable in the UFC. He won the middleweight championship in his second fight with the company and had racked up 16 straight UFC wins heading into his first fight with Chris Weidman.

While Weidman was a big underdog, he was not without momentum himself and headed into the bout at UFC 162 with a perfect 9-0 record. However, few could have anticipated the stunning nature of Weidman's eventual knockout win.

Silva employed his trademark taunting against Weidman in the second round, with his hands down by his side. Weidman was not fazed, however, and closed the distance and finished his legendary opponent with a barrage of punches.

The end of Silva's seven-year winning streak shocked UFC fans everywhere. While less of an upset, the rematch between the two was just as stunning as Anderson Silva unfortunately suffered a broken leg after Weidman checked one of his kicks.

#4. Luke Rockhold vs. Michael Bisping II - UFC 199

Bisping and Luke Rockhold fought for the title at UFC 199

Luke Rockhold and Michael Bisping fought in a rematch at UFC 199 for the middleweight crown.

Rockhold came into UFC 199 on an amazing run. He had won his last five and his only UFC loss came via a freak spinning heel kick from Vitor Belfort. He had knocked out Chris Weidman to win the title in his last fight and appeared set for a lengthy reign atop the division.

Bisping was a huge underdog, taking the fight on 17 days notice and appearing to be towards the end of his career at 37 years old. It did not help that he had also lost before to Rockhold. However, a stunning left hook from Bisping saw him earn a knockout victory and become champion in a shocking upset. Thanks to Bisping's witty banter in the build-up to the fight, and his legendary heart and determination, his knockout of Luke Rockhold to win the title will not be a fight fans soon forget.

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#3. Ronda Rousey vs. Holly Holm - UFC 193

Holm upset Rousey in just her third UFC fight

Ronda Rousey is the biggest star women's MMA has ever seen. Heading into her fight with Holm, she appeared to be at the height of her powers. Rousey had a perfect 12-0 record and had finished each of her last four opponents inside the first round.

Holly Holm had some momentum of her own at 9-0 but had hardly set the world on fire, earning one split decision and one unanimous decision win in her first two UFC fights. This led to most anticipating another routine victory for Rousey.

However, the fight saw Holm turn the world upside down with a second round KO victory. Holm was circling Rousey constantly, getting the champion off balance before delivering a picture-perfect head kick that sent the MMA world spiraling.

According to the oddsmakers, Holm's stunning upset of Rousey was the single biggest upset in UFC history.

Watch Holly Holm react to her stunning UFC 193 victory here:

#2. Amanda Nunes vs. Julianna Peña - UFC 269

Peña was 2-2 in her last four prior to fighting Nunes

UFC 269 saw Julianna Peña's upset of Amanda Nunes immediately enter the conversation as one of the most shocking victories in UFC history. Nunes is regarded as the greatest female fighter of all time and most felt this bout was going to be another easy victory for 'The Lioness'.

Nunes labeled Peña delusional for believing she would be able to pull off the upset. 'The Lioness' certainly seemed to be on the way to another victory after a strong showing in the first round. The second round, however, was a completely different story.

Peña rocked Nunes on the feet before locking in a rear-naked choke to win her first UFC championship. The result stunned fans everywhere and is easily the biggest upset of recent times.

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#1. Georges St-Pierre vs. Matt Serra - UFC 69

Matt Serra was inducted to the UFC hall of fame in 2018

Georges St-Pierre is regarded by many as the sport's GOAT but it can't be forgotten that his legacy includes being on the receiving end of one of the most shocking moments in the company's history.

St-Pierre had only ever lost to Matt Hughes in his career and had finally righted that wrong in his last fight to become welterweight champion. Serra, on the other hand, had only managed a 5-4 record in his UFC career to that point.

In fact, Serra only earned the title shot thanks to winning season four of The Ultimate Fighter. In a change to the traditional TUF format, Serra's success earned him a guaranteed title shot which just happened to be against Georges St-Pierre.

With everything pointing towards a comfortable GSP victory, Serra pulled off arguably the biggest upset in UFC history when he knocked out the reigning champion in the very first round.

The combination of St-Pierre's greatness, Serra's lack of credentials at that point in his career and the nature of how Serra earned the opportunity all combined to make this arguably the greatest upset in the history of the UFC.