ROME -- Only three years ago, the world was a vastly different place. The same was true in professional tennis, maybe more so.

Iga Swiatek, ranked No.15 among Hologic WTA Tour players, and No.35 Coco Gauff tore through the 2021 Internazionali BNL d’Italia field and collided in the semifinals. They were, respectively, 19 and 17 years old.

Swiatek knocked off Barbora Krejcikova in the Round of 16 -- the player who would go on several weeks later to win the title at Roland Garros -- and two-time Rome champion Elina Svitolina in the quarterfinals. Gauff defeated No.4-ranked Aryna Sabalenka in the Round of 16 and No.1 Ashleigh Barty in the quarters. It was the first time they ever met in a match.

Swiatek prevailed over Gauff 7-6 (3), 6-3 and would go on to obliterate Karolina Pliskova 6-0, 6-0 in the final.

On Thursday, slightly older and significantly wiser, Swiatek and Gauff meet at the same stage in Rome as two of the top-three ranked players in the world. They are also, statistically speaking, the best server (Swiatek) and best returner (Gauff) on tour at the moment.

In the other semifinal, it’s No.2 Aryna Sabalenka versus No.13 Danielle Collins. Excluding the WTA Finals, this is the first time the WTA Tour’s top three players have reached the semifinal at the same event since Roland Garros in 2013.

Here’s a closer look at the four semifinalists:

The Case for Swiatek

Do we really need to make a case for the World No.1? OK, fine.

She’s won 10 straight matches on red clay and is gunning for her third Italian Open title in four years. The Madrid-Rome double only would be history’s third, after Dinara Safina in 2009 and Serena Williams in 2013.

Swiatek hasn’t dropped a set here yet and, in fact, has lost only 21 games in four matches. Her serve is working, her decision-making has been exceptionally clean and -- this just in -- she’s clutch under pressure. Even when opponents manage to score a break opportunity, Swiatek’s somehow saved 19 of 23 points.

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How far ahead is Iga ahead of the field?

Swiatek’s 6-1, 6-3 win over Madison Keys in the quarterfinals lifted her rankings point total to 11,085, equaling her highest. The only other player to exceed 11,000 points was Serena Williams, the acknowledged GOAT of modern-era tennis.

Her biggest advantage over Gauff is a hulking 9-1 head-to-head record. It’s 3-0 on red clay, which in a back-to-the-future kind of way includes that 2021 semifinal.

The case for Gauff

It seems she’s been around for so long, you sometimes forget that, relatively speaking, she’s still a kid. This might be a good thing, considering that epic head-to-head mark.

Gauff has been going about her business here, defeating Paula Badosa in the fourth round (in three sets) and taking the first meeting ever with Zheng Qinwen 7-6 (4), 6-1. The serve is, admittedly, still a work in progress. She’s had 41 double faults in four matches, but only six came against Zheng.

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“It feels like a new start for me,” Gauff said of this match against Swiatek. “This is my first time playing her on clay since Roland Garros last year. I feel like a different player.

“I’m coming into these matches with a lot of confidence. If I can go into the match playing like I did tonight [against Zheng], win or lose, I think I have a lot to be proud of.”

Some context for what she’s doing: Gauff has now won more WTA 1000 matches than anyone aged under 21; she just passed Caroline Wozniacki. Moreover, Gauff has reached six WTA 1000 semifinals before her 21st birthday, one more than Swiatek had achieved. Only Wozniacki (nine) had more.

The case for Sabalenka

The road through Stuttgart, Madrid, Rome and Paris is long and winding and, for the better part of a day, that journey almost caught up with Sabalenka.

She tweaked something in her lower back in the quarterfinal match against Elina Svitolina and required medical assistance before winning a third-set tiebreaker. Sabalenka was down 2-0 in the final frame and wound up saving three match points.

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One thing Sabalenka doesn’t get a lot of credit for is her toughness. During that medical timeout, Tennis Channel reported that in 434 career WTA Tour matches, both qualifying and main draw, Sabalenka had never retired during a match. Roger Federer played some 1,500 matches and, famously, never retired.

Make that 435 for Sabalenka. After skipping practice on Tuesday and testing her back with a light hit on Wednesday morning, Sabalenka pronounced herself fit. The swiftness (Swiftness?) of her 6-2, 6-4 victory over Jelena Ostapenko should convince you that she’ll be OK for this one.

Sabalenka holds a commanding 5-0 head-to-head over Collins, including the dramatic Round-of-16 match from Madrid last month. Sabalenka overcame a sluggish start to win 4-6, 6-4, 6-3. It can be argued that kicked her into the form that carried her to the final.

“Yeah,” she told reporters, “that was definitely like a good slap. She played incredible tennis and somehow I was able to turn the game around. If I’m going to face her, hopefully I won’t let her get to the point where she’s almost closing the match.  I’ll start better from the beginning.”

The case for Collins

No one is hotter than Collins after her 6-4, 6-3 victory over No. 24 seed Victoria Azarenka. It was the 30-year-old American’s 19th victory in 20 matches.

She’s announced that this is her last year on the Hologic WTA Tour and, after the match, Collins was asked yet again if she’d reconsider.

“A lot of it has to do with my health -- that’s life,” Collins said. “I’m going out with a bang.”

Earlier this year, Collins put together a 15-match win streak, taking titles in Miami and Charleston. In her fourth appearance in Rome, Collins is into her first semifinal. Only Swiatek (24) has won more matches than Collins (17) in WTA-1000 tournaments this year.

With Gauff, she’s one of two Americans in the semifinals. The last time two Americans advanced to the Rome semifinals was 2016 (Serena Williams and Madison Keys).

“I’ve been practicing well,” Collins said. “I had to just stay dialed in there. I’ve gotten better at hitting the small parts of the court.”

That 5-0 head-to-head is working against her, but there’s something to be said for momentum. No one has more at the moment than Collins.

The best of Collins' baseline barrage vs. Azarenka in Rome