Serena Williams and Ronda Rousey have something in common besides claiming two of the top three spots in The Associated Press' Female Athlete of the Year voting.

They arguably occupied the top two spots in the category of 2015's biggest upsets. But which was bigger?

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Williams was two matches away from becoming the first tennis player to win the calendar Grand Slam in 27 years. In fact, she already was the defending champion in all four Grand Slam events when she took on then-No. 41 Roberta Vinci in the semifinals of the U.S. Open.

Serena won the first set in relatively comfortable fashion, 6-2. But she became increasing tentative as the match went on and visibly was overwhelmed by the pressure of the moment in the third set.

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Vinci, playing with nothing to lose, roared back to win the next two sets 6-4, 6-4 to advance to the finals.

Two points of irony: First, Vinci was scheduled to face revitalized Eugenie Bouchard in the fourth round. But Bouchard, who had won three straight matches for the first time since the Australian Open, suffered a concussion slipping on a wet floor inside a physiotherapy room at the U.S. Open prior to the match and was forced to withdraw.

Second, Vinci lost to countrywoman Flavia Pennetta in the finals, making Williams' upset even bigger.

Ironically, Rousey revealed to ESPN The Magazine that Williams texted her right after Thanksgiving to offer condolences, some two weeks after Rousey's stunning loss to Holly Holm at UFC 193.

Rousey entered that match as much as a 20-to-1 favorite over Holm, who was ranked No. 15 in the UFC a week before their fight, according to mmamania.com.

Holm, unlike Vinci, entered the match with a champion's pedigree; she was a longtime women's boxing champion who made the switch to mixed martial arts.

So while Vinci may have been the less heralded underdog, Holm's claim to bigger upset was the manner in which she dominated and outclassed what had been considered the toughest woman on the planet.

Their fight was so one-sided that it really is a testament to Rousey that she lasted to the second round, although she told ESPN The Magazine that she was out on her feet from a Holm punch in the first 30 seconds of the match.

Point of irony: Rousey had gone on The Tonight Show with Jimmy Fallon and described exactly how Holm would fight her and try to knock her out with a head kick --- but Rousey said she would not let that happen.

But she did, which helps her match edge Serena's as the biggest upset of 2015.

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