With one of the more improbable runs we've ever seen, Emma Raducanu completed the Grand Slam season with a US Open title, where she became the first qualifier in the Open Era to win a major championship. 

Raducanu thrust herself into the tennis spotlight, along with this year's runner-up in New York, Leylah Fernandez. 

While the all-teenage final is a memory we won't soon forget, there were plenty of other unforgettable moments this Grand Slam season. 

What were the best matches? Biggest upsets? And yes, it's early, but what should we expect in Australia?

Our scrupulous writers reveal their final thoughts on a wild ride from Australia to Paris and London to New York. 

Most impressive run to the title

Juzwiak: Each of this year’s Grand Slam singles champions had impressive runs, but Emma Raducanu’s path to the US Open title stands out for me. Not only did she sweep to victory without dropping a set in just her second Grand Slam main-draw appearance, but she did it having also dominated in the qualifying event the previous week. It was a display of confidence and composure well beyond her limited tour-level experience. A qualifier winning a major is an achievement I never thought I would see happen —  and she made it look easy.

Macpherson: The beauty of this year's Grand Slam title runs was that they each captured the imagination in completely unique ways. But I'll go with what Ashleigh Barty accomplished at Wimbledon. She was coming off an injury, in the unenviable position of being World No.1 without holding a major title, and at the tournament she'd dared to publicly dream of winning. The Australian went through a draw that included Roland Garros champion Barbora Krejcikova and, in a pivotal semifinal, resurgent former Wimbledon champion Angelique Kerber to elevate herself to multiple Slam titlist.

The level-headed Barty is a player whose sensible, process-driven approach doesn't leave much room for grandiose ambition. At SW19, she set her sights on a tournament trophy from the start - and pulled it off while remaining entirely true to herself.

Photo by Garrett Ellwood/USTA

Nguyen: Osaka’s was the most dominant. Barty’s the most emotional. Raducanu … I’m still trying to understand what we just witnessed. But for “impressive,” I have to go with Barbora Krejcikova. The 25-year-old did not have the benefit of the innocent, fearless, audacity of youth or the experience of accomplished champions as she marched her way to an improbable Roland Garros title, her maiden major. Coming off her first WTA singles title in Strasbourg, Krejcikova beat the likes of Elina Svitolina, Sloane Stephens and Coco Gauff, saved match points against Maria Sakkari, and edged Anastasia Pavlyuchenkova 6-4 in the third to win the French Open.

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To top it all off, she won the doubles to become the first woman to sweep Paris since 2000. A year ago, Krejcikova, 25, had yet to crack the Top 100 in singles, and it wasn’t for lack of trying. In a career-altering decision, Krejcikova skipped the US Open doubles event last fall to stay in Europe and play ITFs and earn points. Her gambit worked. She earned direct entry into Paris, made the Round of 16, cracked the Top 100 and hasn't looked back.

Top overall match

Juzwiak: Apologies to Maria Sakkari, but Barbora Krejcikova’s whisker-thin 7-5, 4-6, 9-7 Roland Garros semifinal victory over the Greek is still my most memorable and impressive Grand Slam match of the year. With momentum seesawing back and forth, both players acquitted themselves marvelously. Krejcikova slammed a backhand winner out of the air to save a match point, Sakkari continued to fight until the bitter end, and after 3 hours and 18 minutes, the Czech thrillingly triumphed. Bonus points are awarded for the fact that, after this win, Krejcikova has blossomed, becoming not only a Grand Slam singles champion but also one of the most consistently strong singles players of the year.

Macpherson: Leylah Fernandez alone provided a plethora of potential choices at the US Open, with her upset of Angelique Kerber particularly scintillating. In the second week of Roland Garros, it was thrilling to see a succession of matches that reminded us that sporting quality doesn't require star names, particularly Tamara Zidansek's epic defeat of Paula Badosa. But shot for shot for a glorious 3 hours and 18 minutes, nothing compares to Kerber's 7-5, 5-7, 6-4 triumph over Sara Sorribes Tormo in the second round of Wimbledon.

Every point was played as if it was the last - extended to gasp-inducing lengths thanks to the pair's athleticism, raised to dizzying heights. The No.2 Court crowd, in a state of some delirium by the time it ended, gave both players a standing ovation afterward and continued that outside as fans lined the gangways to applaud Kerber and Sorribes Tormo back to the locker room.

Nguyen: My two standout Slam matches were a pair of avoided upsets from match point down. What happens at the Australian Open if Garbiñe Muguruza converts one of the two match points she had on Osaka in the Round of 16? Down a break in the second and third sets, Osaka elevated each time to edge the Spaniard. Most memorably, she hit three unforced errors to give the Spaniard two match points and proceeded to play the next 22 points without making an unforced error, winning 4-6, 6-4, 7-5.

More recently, Karolina Pliskova's late-night slugfest against Amanda Anisimova in the second round of the US Open was a revelation, both for Pliskova and Anisimova. Despite serving a career-best 24 aces, Pliskova was getting pushed off the baseline by Anisimova’s audacious shot-making and power. The partisan crowd on Ashe was going absolutely wild with every Anisimova winner, but Pliskova proved her veteran mettle in saving a match point in the deciding tiebreak to win 7-5, 6-7(5), 7-6(7).  

Most notable upset

Juzwiak: After a couple of rounds where the formbook held up surprisingly well, Leylah Fernandez blew the roof off of Arthur Ashe Stadium with a come-from-behind upset of defending champion Naomi Osaka in the US Open third round. Osaka had won four of the past six hardcourt majors and is always a threat on this surface, but Fernandez used sterling returns and impeccable timing to grit out the second set from a break down and turn the match around. Just as notably, Fernandez’s upset of Osaka proved not to be a one-off, but rather the first indicator of the trailblazing final that electrified New York on Saturday.

Macpherson: 'Notable' can mean a few things - the birth of a new star, the most surprising. But the biggest sliding-doors moment for me was Tamara Zidansek's 7-6(1), 6-7(2), 9-7 upset of Bianca Andreescu in the first round of Roland Garros. It wasn't necessarily a shock, given Zidansek's underrated clay-court skills and Andreescu's lack of form. But it came down to the wire - and if the Canadian had pulled it out, the route Zidansek ended up taking all the way to the semifinals would have been hers. Andreescu has been particularly luckless in 2021, with injury, illness and unfortunate draws all combining to prevent her gaining any match momentum. This match could have changed that.

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Nguyen: Shelby Rogers getting her first win in five tries over No.1 Barty, coming back from a double-break down in the third to win 6-2, 1-6, 7-6(5). After winning Wimbledon and Cincinnati, there was as close to an air of inevitability around Barty as we’ve seen in a top seed at a Slam. As Alex mentioned, we talk a lot about “sliding door” moments throughout the season, and this felt like a big one. Had Barty closed out the win, she would have faced Emma Raducanu in the next round.  

Favorite moment 

Juzwiak: Carla Suárez Navarro has always been easy to root for, armed with an eye-catching single-handed backhand and a positive demeanor. In 2021, the Spaniard wrapped up her career as one of the sport’s ultimate warriors, playing exciting matches in her final events just months after defeating cancer. Despite receiving a terrible draw, it was so gratifying to see her play her final Wimbledon singles match in front of the Centre Court crowds, against World No.1 Ashleigh Barty in the first round. She took a set off the eventual Wimbledon champion, because she’s a fighter, on and off the court.

Macpherson: I consider myself a Hsieh Su-Wei aficionado, having followed the most idiosyncratic player on tour ever since the 37-match winning streak with which she began her career as a 15-year-old in 2001. It's been quite a ride, but even now there's always a new and extremely weird factoid to learn about her. As she carved her way to her first Grand Slam quarterfinal at the Australian Open, coach Paul McNamee revealed to the media that Hsieh experiences broken strings so rarely that she plays with the same racquet for years on end and is so unfamiliar with the feeling that she once didn't even realise it had happened. Mind blown.

Nguyen: Ons Jabeur putting on an absolute show for the Wimbledon Centre Court crowd in a 5-7, 6-3, 6-2 win in the third round. I mean ... come on.

Way-too-early Australian Open prediction

Juzwiak: It’s fair to say 2021’s majors each concluded at one extreme or another, with heavy favorites winning (Osaka, Barty) or with surprise champions who very few people, if anyone, predicted to win (Krejcikova, Raducanu). Prognostication at this point seems to be a fool’s errand — but as a fool, I will guess that Ashleigh Barty, this year’s most prolific titlist, will thrill her homeland fans and move one step closer to a career Grand Slam. Also, the Melbourne crowds should take note from the roaring fans in New York. They can nudge Barty that extra bit in her most challenging matches to help her triumph on home soil.

Rankings watch: Raducanu, Fernandez reach career highs; Hsieh returns to doubles No.1

Macpherson: There will be so many compelling narratives heading into Australia. Can Ashleigh Barty succeed in her quest for a home Grand Slam title? Can Angelique Kerber's renaissance last into 2022? Will Bianca Andreescu and Naomi Osaka regroup and rebound from their tumultuous seasons? I'm particularly eager to see how Emma Raducanu and Leylah Fernandez's success inspires their peer group - specifically, Marta Kostyuk and Clara Tauson, who would have been considered on much the same level as the US Open finalists a fortnight ago. Personally, I have faith that Andreescu will come good again - as long as her health allows.

Nguyen: My only prediction at this point is the pressure on Ashleigh Barty will be incredibly high. Oh, and the weather will be hot.