Half of the Australian Open final 16 is set, with three tantalizing encounters between seeded players headlining Sunday's action. Such is the depth of the WTA Tour that collisions of legends, winning streaks and former World No.1s can occur at this stage of a tournament. And even more remarkably, two of the most intriguing are first-time meetings. Here's what's at stake: 

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[7] Aryna Sabalenka (BLR) vs. [10] Serena Williams (USA)

Williams, holder of the Open era record of 23 major titles, meets the surging Sabalenka, who has collected more trophies than anyone else since Roland Garros. Neither has lost a set yet this fortnight, though Williams had to save two set points against Anastasia Potapova in the third round.

Both have something to prove on the Grand Slam stage. Sabalenka fell to Kaia Kanepi in her Gippsland Trophy opener last week, ending her 15-match winning streak, which included titles in Ostrava and Linz at the end of 2020, and in Abu Dhabi to kick off 2021. Yet, his week marks only her second run to the fourth round of a major, following the 2018 US Open, where she lost at that stage to eventual champion Naomi Osaka in a tough three-setter.

The gulf between Sabalenka's performances at Grand Slams and on the WTA Tour can partly be explained by tough draws. Among those, Sabalenka fell to Amanda Anisimova (at both the Australian and French Opens in 2019), a resurgent Victoria Azarenka (US Open 2020), Magdalena Rybarikova on grass (Wimbledon 2019), upset artist Yulia Putintseva (US Open 2019) and veteran Carla Suárez Navarro (Australian Open 2020).

There are few more formidable obstacles than Williams, who remains in pursuit of a 24th Grand Slam title and first since becoming a mother. Williams, 39, a finalist at four of the last nine majors, is 53-8 at this stage and 12-3 at the Australian Open with her only losses coming to Elena Likhovtseva in 2000, Ekaterina Makarova in 2012 and Ana Ivanovic in 2014.

Sabalenka has been open about being inspired by Williams growing up: "I think I said, I want to hit even stronger than her," she recalled about watching the American. "I don't know if it was smart or not, but at that point I was thinking, wow, she's really powerful. I want to be powerful, too, and I want to dominate on tour the same [as she does]."

That was when Sabalenka was 13. Nine years on, the 22-year-old will finally get a chance at testing just how powerful she's become when she meets Williams for the first time, with a Grand Slam quarterfinal debut at stake.

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[14] Garbiñe Muguruza (ESP) vs. [3] Naomi Osaka (JPN)

Arguably an even more surprisingly blank head-to-head is that between two former World No.1s,  the two youngest active multiple Grand Slam champions. Muguruza and Osaka account for five major trophies combined since 2016, but their careers have seemed to exist in parallel rather than becoming a rivalry proper.

Neither have won a major in the same year or at the same venue: Muguruza captured Roland Garros 2016 and Wimbledon 2017, both when Osaka had yet to crack the Top 50. The Japanese player made her move into the elite the following year and has won two US Opens and one Australian Open since. Concurrently, Muguruza's consistency dipped and has fallen out of the top 10. 

That is in large part due to COVID-19, as the lack of events haven't given players as much of a chance to climb back up in the rankings changes.

Muguruza's run to last year's Australian Open final marked her return to the highest level of the sport. Including her Yarra Valley Classic final showing last week, Muguruza, 27, has conceded four or fewer games in all seven of her wins on Australian soil and has dropped only 10 games this week to make the second week of a major for the 15th time (and sixth at the Australian Open).

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But the last piece missing from the Muguruza resurgence is a trophy. The World No.14 lost to Sofia Kenin in Melbourne last year and to Ashleigh Barty in the Yarra Valley title match. Muguruza's last title was Monterrey 2019, and her most recent above 250 (formerly International) level was Cincinnati 2017. And for all Muguruza's recent ruthlessness, Osaka has begun to quietly dominate at another level. The 23-year-old has not lost a match in over a year. She owns a 33-3 record since September 2019 and has dropped only 13 games in three matches this week, all against former Grand Slam quarterfinalists, Anastasia Pavlyuchenkova, Caroline Garcia and Ons Jabeur.

The last 16 is something of a talismanic round for Osaka. She has reached this stage at five previous majors and each time she has passed it she has gone on to win the title. Both Osaka and Muguruza own a narrowly positive fourth-round record, with Osaka 3-2 (1-1 at the Australian Open) and Muguruza 8-6 (2-3 at the Australian Open).

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[15] Iga Swiatek (POL) vs. [2] Simona Halep (ROU)

Another season, another Grand Slam last-16 clash between Swiatek and Halep, who have yet to play each other at any other stage of any other tournament. The intrigue of this edition of their burgeoning rivalry, though, is that it is the first away from clay, the preferred turf of both players.

The pair's two previous Roland Garros duels serve primarily as indications of just how steep Swiatek's learning curve has been since emerging on to the main tour just two years ago. In 2019, an inexperienced and hampered rookie in the second week of a major for the first time, the Pole lost quickly to defending champion Halep 6-1, 6-0 in 45 minutes. Sixteen months later, it was her turn to deal out the rout, avenging her loss 6-1, 6-2 in 67 minutes en route to her maiden Grand Slam title.

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How will their games match up on fast hard courts? Halep is a former Australian Open runner-up (to Caroline Wozniacki in 2018) and owns a 14-4 record at this stage of majors (4-1 at the Australian Open, with her only loss coming to Serena Williams in 2019). 

But Swiatek came within two games of making her own Grand Slam quarterfinal debut here last year, losing a 6-7(4), 7-5, 7-5 heartbreaker to Anett Kontaveit and a trio of straight-sets wins this fortnight have indicated that when it comes to playing with the expectations that come with being a Slam champion, she's racing up that learning curve as quickly as she always has.

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Hsieh Su-Wei (TPE) vs. [19] Marketa Vondrousova (CZE)

Just  a month ago, in the first round of Abu Dhabi, Hsieh served notice that her singles form was on the uptick by squeezing past Vondrousova 3-6, 6-3, 7-6(3).

That should have been a foreshadowing of Hsieh's exploits this fortnight. Turning around her 1-7 singles record in 2020, she has thrilled the crowd with wins over Tsvetana Pironkova, Bianca Andreescu and Sara Errani to reach the fourth round of a major for the fourth time. Three of those have now been at the Australian Open. Remarkably, the first came in 2008 as a qualifier ranked World No.158, a run that enabled Hsieh to break the Top 100 for the first time.

That was seven years after Hsieh reached the semifinals on her WTA debut at Bali in 2001 as a 15-year-old. The Chinese Taipei player's career arc has been as idiosyncratic and unpredictable as her game. Having gained a reputation as a giant-killer in her thirties (Hsieh had famously lost her first 14 matches against Top 10 opposition, but since 2017 has won seven of her past 10), it would be fitting if the former teenage prodigy finally sealed her Grand Slam quarterfinal debut at the age of 35. If Hsieh accomplishes this, she will become the oldest player - male or female - to debut in the last eight of a major in the Open Era.

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Hsieh's opponent has much to prove as well. Vondrousova arguably ushered in the latest wave of elite teenagers on tour, becoming the first player in a decade to reach a Grand Slam final before turning 20 at Roland Garros 2019. But the Czech has slipped back into the shadows since. A month after her breakout turn, she was sidelined for six months due to left wrist surgery. In 2020, she failed to regain her 2019 momentum amidst the COVID-19 pandemic.

Back in the second week of a major for the first time since that Parisian run, the 21-year-old has slipped under the radar so far despite a run to the Yarra Valley Classic semifinals last week. For Vondrousova, this spells an opportunity to step back into the spotlight.