Upsets are a part of the daily tennis narrative. They happen all the time, some obviously more impactful than others.
But then there are those upsets, the stunners that leave an indelible mark on a tournament and often the season.
In 2021, a string of shocking results rocked the US Open, where two teenagers pulled off major wins to not only change the trajectory on the event, but their careers as well.
Here's a look back at a memorable two weeks in New York, which showcased one upset after another, while setting the table for one of the most improbable finals to date.
Leylah Fernandez def.  Naomi Osaka, 5-7, 7-6(2), 6-4, US Open third round
While Emma Raducanu methodically worked her way through the top half of the US Open draw, Leylah Fernandez blazed her own path in the bottom half at the year’s final major. A third-round upset over defending champion Naomi Osaka made viewers take notice.
Despite a higher ranking than Raducanu (No.73 to the Brit’s No.150), Fernandez, unlike Raducanu’s breakthrough fourth-round showing at Wimbledon, had not posted a signature summer result coming into New York. Between her first WTA singles title, at Monterrey, and the US Open, Fernandez had a ho-hum win-loss record of 7-8 on the tour level. However, the Canadian teenager found a rich vein of form in Flushing Meadows in time for her battle with the reigning champ Osaka.
Osaka had played sparingly over the summer, but she had won four titles over the past six hardcourt Grand Slam events. Expectations were high that the World No.3 Osaka could play her way into form, and she took the first set without facing a break point. But Osaka failed to serve out the match at 6-5 in the second set, and Fernandez suddenly turned the affair around with her first break of the day. From there, Fernandez stayed mentally sturdy, cruising through the second-set tiebreak and leading through the entire third set to complete the stunner.
For the rest of the event, Fernandez stirred the crowds into a frenzy behind her pristine timing and fierce determination. Fernandez’s comeback win over Osaka was the first indicator of the fortnight that a game-changing major was afoot in New York. More upsets by the Canadian would follow. -- Jason Juzwiak
[Q] Emma Raducanu def.  Belinda Bencic, 6-3, 6-4, US Open quarterfinals
Hindsight is a wonderful thing when it comes to contextualizing upsets. Sofia Kenin lost to the unranked Olivia Gadecki at the Phillip Island Trophy - but then needed her appendix removed a couple of days later. Ashleigh Barty fell to World No.71 Paula Badosa in Charleston - and by the end of the year, Badosa was in the Top 10.
Three months after Emma Raducanu's upset of Belinda Bencic in the US Open quarterfinals, it's still shocking. After a raft of higher-ranked players had fallen, Bencic was arguably the favorite to win her first Grand Slam at that stage. Freshly crowned as Olympic champion, her Tokyo triumph had felt like a real breakthrough for a player who had often seemed burdened by expectations following her dominant junior career. Capturing one of the biggest prizes in tennis should have been both an injection of self-belief and a lifting of that weight.
As for Raducanu, she had already enjoyed a fairytale run as a qualifier to the quarterfinals. The 18-year-old had yet to face a Top 40 opponent in her career, though, so all logic pointed toward a confident, in-form Bencic ending it there. The manner in which the match unfolded was as jaw-dropping as the end result. Raducanu came out swinging freely and played a thoroughly composed game. Bencic started well but seemed to freeze up when it became apparent that Raducanu's level wasn't dipping.
For Raducanu, it would be the most significant victory en route to a historic Grand Slam crown. For Bencic, the loss resurrected all of the questions about her ability to rise to the challenge of a major that the Olympic gold medal should have laid to rest. Will the 2021 US Open always be "the one that got away" for the Swiss? -- Alex Macpherson
Leylah Fernandez def.  Aryna Sabalenka 7-6(3), 4-6, 6-4, US Open semifinals
I agree with Jason that the win over Osaka was Fernandez's marquee win of a seed-slaying fortnight, but her semifinal victory over Sabalenka laid to rest any doubts about whether the diminutive 19-year-old could withstand a pure barrage of power. From a pure tennis perspective, I didn't see it coming.
Sabalenka had been a freight train through her first five matches, reeling off nine consecutive sets while vanquishing Nina Stojanovic, French Open semifinalist Tamara Zidansek, red-hot Danielle Collins, former semifinalist Elise Mertens and French Open champion Barbora Krejcikova. In her four matches going into her second major semifinal of the season, the 23-year-old was averaging just five games lost per match.
With her radar locked in and how well she fared in her first major semifinal at Wimbledon earlier in the summer - a three-set loss to Karolina Pliskova - there was plenty of reason to believe Sabalenka owned a heavy advantage over the surely exhausted rookie, who had previously made it past the second round of a major just once in her career.
Yet, by the time Fernandez sealed her spot in her first major final by tallying her third Top 5 win of the tournament, the Canadian cemented herself as an indefatigable battler. After rallying from 4-1 down and saving set point, Fernandez stood tall to take the opening set in a tiebreak. As she went toe-to-toe with Sabalenka deep into the third set, it was the World No.2 who blinked, misfiring on four straight unforced errors to lose the match after 2 hours and 20 minutes.
Frankly, you could probably pick any of Fernandez's four New York upsets - including her three-set battles over Angelique Kerber and Elina Svitolina - and she wasn't necessarily the favorite to beat former Top 20 players Ana Konjuh or Kaia Kanepi in the early rounds, either. But her rousing semifinal victory, which saw Fernandez once again whip the New York crowd into a frenzy, squashed Sabalenka's biggest chance so far of capturing her first major title. -- Courtney Nguyen