The highest-ranked player remaining in the bottom half of the French Open draw is rolling into week two. 

After No.3 seed Aryna Sabalenka was upset early Friday by Anastasia Pavlyuchenkova, No.7 seed Serena Williams overcame a second-set deficit to see off fellow American Danielle Collins in the late afternoon to reach the round of 16, 6-4, 6-4. 

"Was it my best match of the week? I don't know," Williams said after the match. "I felt that I needed to be really focused way a lot in this match because she's playing really well. I've had a rough clay court season thus far, so I'm happy to get some wins on the clay."

In 85 minutes, the 23-time Grand Slam champion was again tested by her compatriot, who took her to three sets earlier this year in Australia at the Yarra Valley Classic: from 4-1 down and 0-15 on serve in the second set, Williams turned the match around to win in straights.

"That felt really good for me," Williams continued. "Things were not going my way. It's not like she gave me those games. I had to earn it and turn it around. That was really positive for me going into the next match.

Trailing 6-4, 1-0 with Williams serving, Collins surged into the form that took her to the quarterfinals at last year's French Open from a set and a break down. she won four straight games to take command of the second set and seemed poised to stretch the match into a decider. 

In a pivotal sixth game, however, Williams earned a hold thanks in part to a pair of early errors from Collins: a backhand that just missed wide, and a forehand error in the net from a commanding position in the rally, which might've seen her lead the game 0-40 as she bid for a third straight break. 

Buoyed by the turn in momentum instead, Williams lost just five points the rest of the way. 

"It was a bit disappointing after being up 4-1. I think Serena played some really great tennis to come back. I don't think I played my best in those moments," Collins said in defeat. "I just have to try to learn from it, hopefully do better next time and be able to close out the set.

"I said [after the match] I'd love to see her win the whole thing and I'd be supporting her. I think we all admire and love Serena, especially the American players. It was pretty surreal today to go out there and be playing against somebody I remember watching at age 9 and 10, playing here, to be here, to be sharing that court with her, this experience. I hope that I can be a steppingstone to her winning another Slam. It would be really exciting and cool, so I'm going to be following every point."

The 23-time Grand Slam champion previously sparkled statistically in a dominant first set, in which she lost just one point behind her first serve and hit 16 winners to nine unforced. The first seven games of the match went with serve, with Collins holding from 0-40 at 1-1 as a moment of note, before Williams secured the decisive break for 5-3.

In all, Williams racked up 22 winners to 20 errors, including five aces, while Collins hit 18 winners to 21 unforced. The unseeded American also landed just 47 percent of her first serves in the match, which afforded Williams the opportunity to win nearly half of the points played in Collins' service games. 

Williams is the only player currently ranked in the Top 15 remaining on the bottom half, followed by No.15 seed Victoria Azarenka, currently ranked No.16. The two former World No.1s could be on course for another chapter in their longtime rivalry in the quarterfinals should they win their fourth-round matches.

Williams will next face No.21 seed Elena Rybakina, as the Kazakh booked a spot in the fourth round of a Grand Slam for the first time with a 6-1, 6-4 win over a resurgent Elena Vesnina - with the latter the beneficiary of a second-round walkover from No.11 seed Petra Kvitova.

The 21-year-old called Williams "the legend of the sport" in her own post-match press conference, and remarked that she, like many, also watched Williams' matches in her youth. 

Read more: Pavlyuchenkova stuns Sabalenka; Rybakina, Zidansek break new ground in Paris

"There's still a lot of matches, a lot of great players, as we can see," Williams said. "There's so much depth in this game now, it doesn't matter if you're playing in the first round or not, you really have to fight for every match and nothing comes easy.

"It's an interesting position to be in because I've been in that position, too, where I've played people that I really admired, but at the same time I wanted to win the match. The tables are turned.

"So now, I feel like that's the same situation, but they want to win. Then they have nothing to lose. They just are hitting lines. You have to just realize that you can hit the lines, too."

Also moving through to the fourth round on Friday were two other seeds, No.20 seed Marketa Vondrousova and No.33 seed Paula Badosa, who will next face each other for a place in the last eight.

The Czech left-hander and 2019 finalist ousted Polona Hercog, 6-3, 6-3, while Badosa reached the fourth round for the second consecutive year after saving a match point to beat Romania's Ana Bogdan, 2-6, 7-6(4), 6-4.

"I'm really proud of myself. I think it was a very tough one, mentally was very tough. I'm proud that I could do the comeback. It was quite tough. The third set, 4-2 down, as well, she was playing very good," Badosa said.

"In one point of the match, I didn't know what to do. I just know that I had to keep that level and keep fighting until the last moment. Sometimes when the opponent plays like that, you just have the option to keep fighting. That's what I did."

Vondrousova lost the first set she played this week against unseeded and looming former World No.15 Kaia Kanepi in the first round, but after winning the last 10 games of that match in an eventual 4-6, 6-3, 6-0 win, she's dropped just 10 games in four sets since. She and Badosa have never played as professionals. 

"I had some pretty good matches this week. I'm just happy with my game and we'll see how it goes, but I'm happy to be in the second week, of course," Vondrousova said. "It's not I think the best tennis I can play, but it's not even the worst one. I think it's the middle.

"I think I'm just very calm in the game, you know. Even though I'm not playing maybe the greatest tennis, I stay calm and I'm just focusing on my game. Eventually I'm winning the matches, so it's kind of important for me also."

Last fall as the defending finalist in Paris, Vondrousova was beaten in the first round by eventual champion Iga Swiatek, winning just three games.

However, due to the changes in the ranking system brought about by the COVID-19 pandemic, the points from Vondrousova's runner-up showing two years ago are still on her ranking until after this fortnight, a fact she herself has acknowledged has been playing on her mind. 

"It's important to have nice draw, of course. If you want to do good in a tournament, you have to beat so many players. Of course, maybe they are unseeded but they play some good matches also," Vondrousova said.

"But, I mean, I played Iga here, and everybody was, like, Oh, my God, [you lost] first round. I was like, 'She played amazing. Just wait, guys.' And she won the whole tournament. I was, like, Yeah, I told you. So it was kind of funny. I'm just happy that I'm playing like this again.

"It's so much points [to defend] and... everyone is like, 'Yeah, it's gonna drop, it's gonna drop.' I'm like, 'Yeah, just please drop it now and I can play calmly.'

"I'm not looking so much at the draw, to be honest... Badosa is playing great on clay, especially this year, so it's gonna be tough. I'm just going match by match and we'll see.

"But now this week was good, so I'm just focusing and maybe I can do more."