Five years of persistent, mysterious pain was suddenly solved when a friend of Danielle Collins recognized the sometimes debilitating symptoms of endometriosis.

And so, the 27-year-old American withdrew from the Volvo Car Open on April 2 – and underwent surgery three days later. Doctors made four separate incisions in the abdominal/pelvic region and removed a cyst Collins described as the size of a tennis ball.

Eight weeks later, Collins defeated Wang Xiyu 6-2, 4-6, 6-4 in the first round of the French Open.

“Before I went out on the court today I was just like, `I have to give it my best, and I have to know my strengths and I have to be aware of the things that might not be feeling great at certain points in time,’” she told reporters afterward. “I was a bit nervous, too, today going out since this was my first real match since surgery.

“I do still think sometimes I was a bit hesitant, but as the match continued to go on, I became more and more confident physically.”

Collins carried that momentum into the second round, emphatically defeating Anhelina Kalinina 6-0, 6-2. Her reward? A third-round match Friday with 23-time Grand Slam champion Serena Williams.

“It certainly presented its challenges, but it’s been really kind of shocking, like since surgery I’ve just felt so much better,” Collns said, “especially with my back pain. Like I’m not having any type of sciatic nerve pain, which I was dealing with for a couple of years. That to me, I always thought it was more joint-related.

“It’s just been a real weight that’s been lifted off my shoulders since having the surgery because I kind of just in some ways got used to it and kind of thought that that was normal to be dealing with. I didn’t realize that I would feel this much better after surgery, so I’m really relieved.”

Suddenly, the bottom half of the draw has opened up considerably. Five of the seven Grand Slam champions didn’t get past the second round. The only ones left are Serena and Victoria Azarenka, who could meet in the quarterfinals.

Here’s how the bottom half looks heading into Friday’s action:

Third quarter

No.7 Serena Williams vs. Danielle Collins

Based on most indicators, this isn’t a lock for Serena.

While struggling in the second set, she ultimately prevailed over Mihaela Buzarnescu 6-3, 5-7, 6-1. Collins, meanwhile, dropped only two games against Kalinina.

And then there is the head-to-head – a single match, contested only four months ago. It was a three-setter at the Yarra Valley Classic, won by Serena, but decided by an unusual 10-6 super-tiebreaker.

Collins, ranked No.50, is a solid 12-5 for 2021 despite her two-month sabbatical following surgery. She reached a career-high of No.23 in January 2019 after making the semifinals of the Australian Open. Just eight months ago, she was a quarterfinalist at Roland Garros, beating 2016 French Open champion Garbiñe Muguruza along the way.

Head-to-head: 1-0, Williams.

No.21 Elena Rybakina vs. Elena Vesnina

On paper, anyway, this all-Elena confrontation is a mismatch.

Rybakina is 21 years old and ranked No.22 in the world. Vesnina is a 34-year-old mother ranked No.1,096 – and has played three singles matches in three years.

But there’s a reason they play these games; the two have never met and who knows how it will turn out?

Rybakina defeated Nao Hibino 6-3, 6-1 in 63 minutes.

Vesnina advanced to the third round via walkover when No.11 Petra Kvitova pulled out with an ankle injury. She dropped only one game in her first-round match.

No.15 Victoria Azarenka vs. No.23 Madison Keys

Neither player had much of a clay warmup before Roland Garros, but here they are, looking at a spot in the Round of 16.

Azarenka played only a single match on clay coming in – beating Ekaterina Alexandrova in Madrid before withdrawing with a back injury. Keys lost three of four in Charleston, Madrid and Rome.

Tuesday, Azarenka handled Danish teenager Clara Tauson 7-5, 6-4, and Keys looked sharp, beating Canadian teenager Leylah Fernandez 6-1, 7-5.

Keys said she was looking forward to the match, but wasn’t sure if they’d played before.

“I think we’ve only played once,” Keys said, “but I’m not the best at remembering matches so I could be wrong. But I’m really looking forward to it. She’s obviously playing some really impressive tennis the past 18 months. I know it’s going to be a really difficult match, super high intensity.

“I think this is the first time in a while for me to be able to have consistent matches in a row on the same surface. I’m feeling like I’m finding my rhythm a bit more.”

Head-to-head:1-0, Azarenka (2018 Miami, Keys retired down 6-7 (5), 0-2).

No.3 Aryna Sabalenka vs. No.31 Anastasia Pavlyuchenkova

Pavlyuchenkova was having a marvelous run last Month in Madrid when she ran into a wall named Sabalenka. She had beaten three Top 20 players – Karolina Pliskova, Jennifer Brady and Karolina Muchova – but Sabalenka sent her home with a 6-2, 6-3 win.

There’s no shame in that; Sabalenka took down world No.1 Ashleigh Barty in the Madrid final. Sabalenka has now won 13 of her 15 matches on clay in 2021 and has marked herself as the player to beat in this bottom half.

On Wednesday, Sabalenka defeated Aliaksandra Sasnovich 7-5, 6-3, while Pavlyuchenkova got past Ajla Tomljanovic 6-2, 6-3.

Head-to-head: 1-1

Fourth quarter

Katerina Siniakova  vs. Tamara Zidansek

Siniakova produced a terrific second-round result, defeating No.29 Veronika Kudermetova in a match that required nearly three hours – and 37 games.

The 25-year-old from the Czech Republic has won five of six matches including a semifinal run in Parma. Ranked No.68, she lost there to eventual champion Coco Gauff, but scored wins against Tauson, Caroline Garcia and Serena. In her last event of 2020, Siniakova reached the final of an ITF event in Dubai, losing to Sorana Cirstea, who she could meet in the Round of 16.

Zidansek, ranked No.85, is a 23-year-old from Slovenia, who qualified in Madrid and pushed Barty to three sets there. She defeated Madison Brengle 6-4, 6-1 in the second round here and stunned No.7 Bianca Andreescu in the first round.

Head-to-head: 2-1, Zidansek (2020 Prague broke a 1-1 tie for Zidansek).

Daria Kasatkina vs. Sorana Cirstea

Quick, name the four players who have won multiple titles in 2021 ... Barty (three), Sabalenka (two) and No.9 Iga Swiatek (two). And? The surprising answer is Kasatkina – ranked No. 37 – who won the Phillip Island Trophy in Melbourne and a month later, the St. Petersburg Ladies Trophy.

Kasatkina, a 24-year-old from Russia, is seeing a career resurgence. Her 20 wins this year are exactly the combined total from 2019 and 2020. And, don’t forget, one of her two Grand Slam quarterfinal berths came at Roland Garros.

“It’s experience and it’s always new to face these kind of matches like quarterfinals and further,” Kasatkina told reporters after defeating good friend and No.10 seed Belinda Bencic 6-2, 6-2. “So in case that I go that far, which will be amazing always, I think for sure this experience would help me.

Kasatkina upsets Bencic in Gen. 1997 round two clash: Roland Garros Highlights

“But as I say, I have a third-round match after tomorrow, and that’s my main goal for the moment.”

Cirstea, at 31, is enjoying a career renaissance, too. She’s won 11 of her past 13 matches on clay, having taken the title at Istanbul (beating Elise Mertens in the final) and reaching the final a week ago at Internationaux De Strasbourg, falling to Barbora Krejcikova. She’s ranked No.54.

It’s been 11 years since Cirstea reached the quarters at Roland Garros.

“Even if it’s been quite a long time since then, I felt always Roland Garros was special,” she said. “I have had a few third rounds where unfortunately I lost to the champion. I think one time to Serena, one time to Li Na. So it was more than that quarterfinal for me when I think about French Open.”

Head-to-head: 1-0, Kasatkina (2018 Madrid).

No.20 Marketa Vondrousova vs. Polona Hercog

Vondrousova likes the red clay of Europe. Two years ago, as a 19-year-old, she ran through to the Roland Garros final, falling there to Barty. A year later, she reached the semifinals in Rome.

For the 21-year-old Czech, consistency has been more elusive. She had lost three of four matches, in Stuttgart, Madrid and Rome coming in, before finding form at Roland Garros.

Photo by Getty Images

She forcefully finished Kaia Kanepi with a 6-0 third set in the first round, then took care of French wildcard Harmony Tan 6-1, 6-3 in the second.

The No.73-ranked Hercog, meanwhile, dispatched French favorite Caroline Garcia 7-5, 6-4 when she converted her fifth match point.

If their only previous meeting is any indication, this could be a wild one. Vondrousova won at 2020 Rome, 1-6, 6-1, 7-6 (5).

Head-to-head: 1-0, Vondrousova.

No.33 Paula Badosa vs. Ana Bogdan

The four best clay-court 2021 seasons so far belong to Top 10 players Barty, Sabalenka, Swiatek – and Badosa. The 23-year-old Spaniard is a phenomenal 15-2 on clay, which includes a win against Barty, a breakout run to the semifinals in Madrid, and a title at the recent Serbia Ladies Open.

Badosa took a 6-2, 6-0 decision from Danka Kovinic, while Bogdan was the beneficiary when No.2 Naomi Osaka withdrew after winning her first-round match.

Bogdan, 28, is trying to equal her best Grand Slam effort, reaching the third round of the 2018 Australian Open.

Head-to-head: 0-0.