French Open 2021: Serena Williams knocked out in fourth round by Elena Rybakina

The fourth round of the 2021 French Open is underway. Serena Williams will look to advance Sunday in a women's field that has seen a number of the top seeds flame out early. On the men's side, big names like Daniil Medvedev, Stefanos Tsitsipas and Alexander Zverev will try to avoid getting upset.

Follow along below for updates on the final day of Round 4 play.

Serena Williams knocked out in fourth round

The women's bracket in the 2021 French Open continues to lose its top players. Serena Williams became the most recent victim of that trend Sunday, losing in straight sets to Elena Rybakina.

Williams struggled early, winning just three games in the first set. She had a better showing in the second set, trading games with Rybakina until she finally pulled ahead late in the set to win 7-5.

With the loss, yet another top women's seed has been eliminated from the French Open. Williams — ranked No. 7 — was among the top women left after Naomi Osaka and Ash Barty pulled out of the French Open and No. 3 seed Aryna Sabalenka was knocked out by Anastasia Pavlyuchenkova.

Sofia Kenin remains the top seed left in the women's bracket. With Williams' loss, Iga Swiatek remains the only other woman ranked in the top-10 to remain in the tournament.

The victory moves Rybakina to the quarterfinals, where she'll take on Pavlyuchenkova. That match will take place Tuesday.

Alexander Zverev cruises after slow start over Kei Nishikori

It took Alexander Zverev a second to get going on Sunday. Once he did, though, the German star had no issue moving out of the fourth round at Roland Garros.

Zverev cruised to a win over Japan's Kei Nishikori in straight sets in their match on Sunday night, 6-4 ,6-1, 6-1.

Zverev jumped up early in the first set, winning the first three games with ease. Nishikori, however, suddenly rallied right back to even up the match. He then tied things back up 4-4 just two games later, too.

That, though, was about it for Nishikori.

Zverev closed out the first set with ease, and then won the first five games of the second set en route to his 6-1 win. From there, he cruised to the win to reach his third quarterfinals in four years at the Grand Slam.

Zverev, the No. 6 ranked men's player in the world, will take on Spain's Alejandro Davidovich Fokina in the quarterfinals.

Daniil Medvedev dominates in win over Cristian Garín

Daniil Medvedev looked like an unstoppable force Sunday. Medvedev dominated on the court, beating Cristian Garín to advance to the quarterfinals of the 2021 French Open.

Medvedev didn't show any signs of vulnerability until the third set. He easily won the first two sets, 6-2 and 6-1. The final set was closer, as Garín managed to take 5 games. Medvedev, however, still pulled out the victory, winning the third set 7-5.

Medvedev dominated on serves, notching 14 aces. Garín did not register a single ace during the match.

The win sets up one heck of a match in the quarterfinals. Medvedev, ranked No. 2, will take on Stefanos Tsitsipas, No. 5, to determine who makes it to the semifinals. The two will square off Wednesday.

Stefanos Tsitsipas advances to quarterfinals with win over Pablo Carreño Busta

The favorites in the men's bracket at the 2021 French Open continue to dominate. Stefanos Tsitsipas advanced to the quarterfinals with a win over Pablo Carreño Busta on Sunday.

Tsitsipas — ranked No. 5 — needed just three sets to move past Busta. Tsitsipas took the first two sets easily, winning 6-3 and 6-2. Busta made things difficult in the third set, but couldn't pull out a win in the set. Tsitspias took the third set 7-5, advancing to the quarterfinals.

Tsitsipas has looked dominant throughout the 2021 French Open, dropping just one set in his four matches. His journey for a grand slam title gets more difficult now, as Tsitsipas will face No. 2 ranked Daniil Medvedev in the quarterfinals.

Roger Federer pulls out of French Open to rest

Roger Federer doesn't want to push himself too hard. Federer, 39, pulled out of the French Open on Sunday, citing the need to rest to surgically repaired knee.

In a statement, Federer explained he didn't want to push his body too far after undergoing two surgeries on his right knee in 2020. Federer hinted he could pull out of the event following his win over Dominik Koepfer in the third round. That match lasted three-and-a-half hours.

After it was done, Federer said he would re-evaluate his body to determine whether he could keep playing. With Federer out, Matteo Berrettini will advance to the quarterfinals of the French Open on a walkover.

Anastasia Pavlyuchenkova continues strong run with upset over Victoria Azarenka

Anastasia Pavlyuchenkova doesn't mind being the underdog. Pavlyuchenkova continued her excellent run Sunday, taking down No. 15 Victoria Azarenka to advance to the quarterfinals.

It looked like it would be a familiar story for Pavlyuchenkova — the No. 31 seed — early. Despiet Pavlyuchenkova's success at the French Open, Azarenka appeared to be a significant roadblock. Pavlyuchenkova dropped five of her last six matches to Azarenka coming into Sunday.

The first set made it seem like that would happen again. Azarenka pulled off a 7-5 win in the set, forcing Pavlyuchenkova into a situation where she had to win the final two sets to pull off the victory.

She did just that, beating Azarenka 6-3 in the second set and 6-2 in the final set. The win marks the first time in a decade Pavlyuchenkova reached the quarterfinals at Roland Garros — where the French Open is held.

Pavlyuchenkova has never reached a semifinals during a grand slam. In order to do that, Pavlyuchenkova will have to defeat the winner of Serena Williams and Elena Rybakina. That match will take place Sunday.

Paula Badosa defeats No. 20 Markéta Vondroušová

The women's singles bracket continues to feature big upsets. Paula Badosa pulled off the latest upset Sunday, knocking off No. 20 Markéta Vondroušová in the fourth round of the French Open.

Badosa — ranked No. 33 — needed three sets to win the match. She got out to a strong start, making the first set from Vondroušová 6-4. Vondroušová fought back in the second set, winning it 6-3. That set up a deciding final set.

Badosa rediscovered her stride in the third set, taking down Vondroušová 6-2 to advance.

With the win, Badosa will take on Tamara Zidansek in the quarterfinals. Zidansek defeated Sorana Cirstea on Sunday to advance to the quarterfinals.

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