The last time they held a Grand Slam, it ended with Aryna Sabalenka and Coco Gauff. The 19-year-old Gauff won that US Open final in three sets, the first of what might one day be an expansive collection of major hardware.

Four months later at the Australian Open, while all the other Top 10 seeds have departed, it’s Sabalenka and Gauff again in Thursday’s semifinals -- the collision that, going forward, could become the standard in women’s hard-court tennis. They are both riding ridiculous win streaks.

The No.2-seeded Sabalenka has won 12 consecutive matches at the Australian Open and has reached six straight major semifinals. The last woman to do both was Serena Williams.

No.4 Gauff has won 12 Grand Slam matches in a row. She’s the youngest player to do that since Maria Sharapova -- and the youngest woman to reach the semifinals here since Nicole Vaidisova 17 years ago.

Sabalenka and Gauff combined for a 19-1 record so far this year; the only loss came when No.3 Elena Rybakina defeated Sabalenka in the Brisbane final.

The other semifinal, between No.12 Zheng Qinwen and qualifier Dayana Yastremska, will produce a first-time major finalist. But they’ll find themselves a heavy underdog to either Sabalenka or Gauff.

This could provide a shift in elite women’s tennis, possibly elevating Sabalenka or Gauff above the 27 Open Era players who won a single Grand Slam title. 

For the 25-year-old Sabalenka, defending her title here would create a surge of confidence and give her a rolling start toward regaining the No.1 ranking.

If Gauff were to win, it would be two major titles while still a teenager. For context, consider that both Iga Swiatek and Naomi Osaka were 21 when they won their second Slam. The last teenager to collect a second major? Sharapova, in 2006. The last teenager to do it back-to-back? Martina Hingis here in 1998.

There's even more incentive for Gauff and Sabalenka. The winner will be ranked No.2 after the Australian Open behind Swiatek.

Here’s how these semifinals break down:

No.2 Aryna Sabalenka vs. No.4 Coco Gauff

The case for Sabalenka

No one is playing better -- and it’s not even close. She has yet to drop a set and the lost-games count in five matches is 16.

Sabalenka, one of the players featured in Netflix's "Break Point," has won 80 percent of her first-serve points, better than Gauff’s 72 percent. She’s also converted 60 percent of her break-point opportunities, best among the quarterfinalists.

To be fair, Sabalenka’s had a relatively easy path to the semifinals. Before meeting No.9 seed Barbora Krejcikova in the quarterfinals, the highest-seeded player Sabalenka beat was No.28 Lesia Tsurenko. But Sabalenka thumped Krejcikova 6-2, 6-3 -- in 71 minutes.

At the same time, Gauff had to dig deep to get past Marta Kostyuk. Down 5-1 in the first set, she rallied to win the tiebreak, erasing two set points along the way. Still, it went to three sets and required 3 hours and 8 minutes -- Gauff's longest major match of her career. Her previous four matches averaged only 72 minutes.

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42.9% Win 3
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Sabalenka won the first set of their US Open final handily, then slowly faded. She forced two break points in the opening game of the second set but couldn’t close the deal. After Gauff broke her to take a 3-1 lead, Sabalenka never offered much resistance.

It will be interesting to see how Sabalenka responds here. After losing to Rybakina a few weeks ago, she’s come tearing back in Melbourne. Sabalenka says she’s “super excited” to play Gauff again, that she wants “revenge.” If she’s serious about becoming the best player in the world, she’ll find a way to win.

'I know my dad would be proud of me': Sabalenka on becoming No.1

Sabalenka’s take: “She’s moving really well. Everything you do on court, it’s coming back. So you need to build the point probably couple times in one point to have that -- not like easy shot, but easy shot to finish the point. So that’s why she’s really tough opponent.

I’m just playing point by point, and that’s it, and fighting for every point without overthinking about my dreams, about what I want to do, about how many Slams I want to win and all that stuff. I think I’m more mature, older, whatever you want to call it. But I think it all comes with experience, with tough losses, with tough matches.

The case for Gauff

Three times at the US Open, Gauff came back to win after dropping the first set. It happened again in the quarterfinals, with the teenager looking extremely nervous and tentative. And yet she came back to win that one, too, playing with her “C” game.

Nevertheless, she’ll feel good coming into this one. Not only is Gauff coming off that US Open win, but she holds a 4-2 head-to-head advantage. Only Iga Swiatek and Kiki Bertens have won more matches against Sabalenka (five). Put another way, the only player against whom Gauff has registered more wins is Katerina Siniakova, also five.

While Sabalenka is now 8-0 in major quarterfinals, her record is 2-5 in semifinals. This is worth noting, because Gauff is undefeated. She’s 2-0 (2022 Roland Garros, 2023 US Open). This is the third major semifinal in her past eight appearances.

Because of her age, sometimes history is the best way to measure her success. Gauff is only the second teenager this century to start a season with a 10-0 record; Justin Henin did it in 2001. There’s another precedent as well. This is the first time since 2011 that the previous year’s US Open finalists met again in Melbourne. Kim Clijsters won in New York and repeated that result against Vera Zvonareva in the semifinals.

Gauff’s take: “Obviously Aryna, always a tough match with her. I think she’s playing well this tournament.

“There’s a quote, it’s easier to get to the top, but harder to stay there. The goal is to stay here as long as possible and keep going upwards. I mean, I’m not at the top but I’m up there.”

No.12 Zheng Qinwen vs. qualifier Dayana Yastremska

The case for Zheng

After dropping a first-set tiebreak to Anna Kalinskaya, Zheng regrouped and won 10 of the last 11 games -- and the match, 6-7 (4), 6-3, 6-1. In a larger sense, she created more tennis history for China.

At 21, she’s the youngest Chinese player in the Open Era to achieve a Grand Slam semifinal berth, three years younger than Zheng Jie was when she showed out at Wimbledon in 2008. Zheng is only the third Chinese player to reach this level at a major, joining Zheng Jie and Li Na.

The Hologic WTA Tour’s Most Improved Player will find herself in the Top 10 for the first time next Monday; a win over Yastremska would move her up to No.7.

While Zheng’s path hasn’t been particularly difficult -- she’s the first player in 40 years to reach the semifinals here without facing a Top 50 player -- her game matches up nicely with the hard-hitting Yastremska.

Against Kalinskaya, Zheng ripped 42 winners and had 10 aces, raising her tournament-leading total to 44. Yastremska has 20. Zheng’s percentage of first-serve points won is also better, 79 percent to 71 percent.

These two have never played.

Zheng’s take: “It’s not easy for [Yastremska] to arrive here in semifinal, of course. When I was struggling in the futures before, I already heard her name, big name in WTA, so is already really good player. Doesn’t matter if she come from the qualies. Right now I think the people who arrive to semifinal, they all have super well level in this tournament.

“I have nothing more to say. Just compete.”

The case for Yastremska

The 23-year-old from Ukraine has created more momentum at Melbourne Park than any other semifinalist. In a span of 14 days, Yastremska won eight matches -- equaling the best streak of her career.

Five things to know about Yastremska: From qualifying to the semifinals

After defeating19-year-old Linda Noskova 6-3, 6-4, she’s only the fifth qualifier in the Open Era to reach a Grand Slam semifinal and the second at the Australian Open. And the degree of difficulty has been impressive. Contrasting with Zheng, all of her five main-draw wins have come over players ranked among the Top 50.

Against Noskova -- the upset winner over World No.1 Iga Swiatek in the third round -- Yastremska forged service breaks at 3-all in each set. She’s absolutely crushing the ball. She had 19 winners and 23 unforced errors, while Noskova’s corresponding numbers were six and 21.

Yastremska played the first quarterfinal match, and it was more than an hour shorter than Zheng’s. Yastremska finished her interview in the main press room eight hours earlier than Zheng, who finished up around 11:30 p.m.

With a swift one-day turnaround, advantage Yastremska.

Yastremska’s take: “It’s nice to make a history [as the first qualifier since 1978]. It’s something new for me and for my generation because the last time it happened it was a long time ago. I wasn’t born yet. It's nice. I'm really happy to be in my first semifinals.

“I think that I have a dream since I’m a child, and that’s what was moving me forward no matter what happens. I had a lot of difficult situations, and I don't want to talk about it right now. But for now I can say is that I just, I relaxed. I was putting a lot of pressure on myself, responsibility. Now I just took it everything out of my bag, and I’m trying to enjoy it.”