UFC upsets can happen at any moment, any split second. Anything can happen when fighters step into the UFC octagon. One mistake can lead to a knockout or open a hole for a fighter to take advantage of a weakness, but there are some fights where fans and experts all agree that one fighter has very little chance of walking out a winner. As we said, anything can happen in the UFC so let's take a look at bouts that none of the experts predicted a loss for heavy favorites.

Holly Holmes vs Ronda Rousey

Ronda Rousey arrived at UFC 193 in Melbourne, Australia, as the heavy favorite to defend her title against challenger Holly Holmes. "Rowdy" Ronda was undefeated to this point in the UFC and was so heavily favored that no one anticipated anything less than a dominating victory.

Someone forgot to tell "The Preacher's Daughter" that Rousey was supposed to win the fight because Holly Holmes came out fighting and looked like the favorite right from the opening bell. After dominating the first round, Holmes landed a kick to the head that knocked out the champion Ronda Rousey and Holmes left UFC 193 with the belt after what may be the biggest women's UFC upsets on the record.

Misha Tate vs Holly Holmes

After knocking out Ronda Rousey to win the title, Holly Holmes became the clear favorite in any projected title defense that would come her way. At UFC 196 she put her belt and undefeated 10 – 0 record on the line against number-two ranked challenger Misha Tate. Even though Tate was highly ranked, oddsmakers considered Holly Holmes a clear and daunting favorite.

Holmes held the advantage through most of the fight and was well ahead on the scorecard going into the fifth round. If she could just survive the remaining few minutes of the fight, the judges would award her a decisive victory. Tate knew she was behind on points and pushed for an advantage in the last round. She found an opening and was able to duck a left hand from Holmes and lock in a rear-naked choke.  Despite the odds being against her and being overmatched through four rounds, Tate pulled off the upset to become a UFC champion.

Nate Diaz vs Connor McGregor

UFC 196 hosted yet another major upset on our list as Nate Diaz came into the match on 11 days' notice to shock the UFC world by forcing the seemingly invincible Connor McGregor to give up.

The bout between Diaz and McGregor features two very confident and brash fighters. McGregor had held multiple titles in different weight classes and Diaz was considered a major underdog due to the limited amount of time he had to prepare for the fight. McGregor held an early advantage, but Diaz turned the tables in the second locking in a naked rear choke that forced McGregor to submit, leading to another massive UFC upset.

BJ Penn vs Matt Hughes

BJ Penn and Matt Hughes had three epic bouts against each other over the course of their UFC career, but their final bout may be considered their most shocking. With Penn winning the first fight, and Hughes winning the second, these two top performers entered UFC 123 ready to settle the score.

Over the course of his career, Hughes was at times considered one of the best pound-for-pound fighters in the UFC and came into this fight as the favorite. It didn't take BJ Penn long to prove the oddsmakers wrong as Penn was able to KO Hughes at the 21-second mark of the first round. The quick end to this fight was part of the reason that this victory ranks as one of the biggest UFC upsets in the history of the sport.

Frankie Edgar vs BJ Penn

In 2010, BJ Penn was set to defend his UFC lightweight title at UFC 112 in Dubai against challenger Frankie Edger. Penn, considered by many to be the best fighter in MMA at this time came into the fight as a huge favorite with no expectations of losing to Edger.

This fight would surprise everyone as it went the distance of a full five rounds without either fighter winning via knockout or submission. Once the judges tallied their scoresheets, Frank Edger walked away with the UFC lightweight title belt in an unpredictable upset. Frankie Edger would go on to prove that this victory was no fluke by beating BJ Penn two more times over the course of his career.

Forrest Griffin vs Mauricio Shogun Rua

UFC 76 featured a much anticipated bout between Mauricio "Shogun" Rua and Forrest Griffin. Upon entering the UFC, Rua was considered the best lightweight in the world and his debut against Griffin was expected to showcase why he was a dominant force to be reckoned with.

Rua would show why he was such a dangerous fighter early, scoring a significant number of takedowns, but Griffin persevered through the first two rounds and found an opening in the third. Forrest was able to counter a takedown by Rua and lock in a rear chokehold that forced Shogun to submit spoiling Mauricio's much anticipated UFC debut.

Matt Serra vs Georges St. Pierre

We round out our list in what nearly every fight fan considers the biggest upset in UFC history. The oddsmakers had Georges St. Pierre as an overwhelming -1300 favorite entering the fight. Matt Serra apparently had no chance to defeat GSP and take the welterweight title. In fact, fight commentators spent most of the lead up to the fight discussing future title defenses for ST. Pierre, completely disregarding the bout on hand at UFC 69.

It took less than one round for the 31-year-old Serra to prove every prognosticator wrong by knocking out the 25-year-old champion at the 3:25 mark of the first. St. Pierre came out firing but just couldn't land any significant strikes. Serra delivered a hook to the side of the champ's head that St. Pierre just could not recover from and Serra scored the biggest UFC upset in the history of the UFC.

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