Naomi Osaka, Aryna Sabalenka, Jennifer Brady and Victoria Azarenka highlight an intriguing slate of second-round matches Sunday at the Mutua Madrid Open. Click here for the full order of play.
Here are the six match-ups worth clearing your day for:
 Naomi Osaka vs. Karolina Muchova (1-0)
Last Meeting: Osaka d. Muchova, 6-7(5), 6-4, 6-2, 2020 Cincinnati 2R.
As the World No.2 continues her quest to win a first clay-court title, she'll go up against the Czech's crafty all-court game. Making her Top 20 debut this week, Muchova is playing just her second tournament since her semifinal run at the Australian Open, having taken time off to heal an abdominal injury.
Osaka looked comfortable in her 7-5, 6-2 win over Misaki Doi in the first round, but Muchova will be a good test of her ability to hold her nerve in the face of slices, off-speed balls, and dropshots. A quarterfinalist in Madrid in 2019, Osaka says she's tapping into the confidence she gained on the surface that season.
"Looking back, it's something I'm actually happy about, because when I was playing on clay that year, I didn't feel uncomfortable at all," Osaka said. "And I actually thought the matches that I lost, even though I didn't play in Rome, but the two matches that I lost it was more mental than it was physical. Like, I didn't feel uncomfortable sliding or I didn't feel that tired. So I'm actually really excited to get that mindset back and to start feeling more comfortable on clay."
The winner will face either Maria Sakkari or Anett Kontaveit in the Round of 16.
STAT: Osaka leads the tour in 1st serve points won (76.2%), while Muchova leads the tour in 2nd serve points won (53.1%).
 Aryna Sabalenka vs. Daria Kasatkina (2-1)
Last Meeting: Sabalenka d. Kasatkina, 7-6(5), 6-3, 2021 Australian Open 2R.
There is no more drastic contrast of styles on the schedule than the fourth meeting between Sabalenka and Kasatkina. The Belarusian had a strong week in Stuttgart, making her second clay-court final with strong wins over Simona Halep and Anett Kontaveit. Kasatkina needed more than three hours to get past Irina-Camelia Begu in her Madrid opener.
Sablalenka and Kasatkina have yet to go three sets in a match, but their duels have been decided by tight sets. Sabalenka has won the past two meetings after edging Kasatkina in two tight first-set tiebreaks and then running away with the match.
The winner will face either Victoria Azarenka or Jessica Pegula.
STAT: Kasatkina is bidding for her first Top 10 win since 2018 Roland Garros (d. No.2 Wozniacki).
 Jennifer Brady vs. Jelena Ostapenko (1-1)
Last Meeting: Brady d. Ostapenko, 6-4, 6-0, 2019 Dubai
Brady vs. Ostapenko features power for power, generated in two different ways. Brady snapped her four-match losing streak with a 6-2, 6-4 win over Venus Williams in the first round. Ostapenko needed just 64 minutes to blast past Polona Hercog in her opener.
This will be the first meeting in over two years between Brady and Ostapenko. The Latvian won their only match on clay, which came back in 2017 in Stuttgart qualifying. Ostapenko won that match, 5-7, 6-1, 6-3.
The winner will face either Karolina Pliskova or Anastasia Pavlyuchenkova.
STAT: Ostapenko won 61.9% of her 2nd serve points in her opening match, up from her season average of 42.6%.
She’s bright, opinionated, inquisitive, and the comeback player of the year last year. She’s a devoted mom and her own person... @vika7 ... thank you for revealing yourself to us! ❤️💪@WTA https://t.co/JaG3Cb1EJ7— Chris Evert (@ChrissieEvert) April 29, 2021
 Victoria Azarenka vs. Jessica Pegula (0-1)
Last Meeting: Pegula d. Azarenka, 7-5, 6-4, 2021 Australian Open 1R.
Pegula's opening-round upset over Azarenka at the Australian Open was the springboard for her breakout quarterfinal run in Melbourne. Can the American replicate the result in vastly different conditions?
In Melbourne, Azarenka was coming out of hard quarantine. The limited preparation impacted her readiness, as she struggled physically toward the end of her straight-set loss.
While Pegula has proven her hardcourt prowess, her comfort on clay is still a work in progress. Her solid win over Istanbul champion Sorana Cirstea in the first round was her first in the main draw of a clay tournament since 2019 Prague.
STAT: Of her 21 titles, Azarenka has won one on clay (2011 Marbella). However, she made back-to-back Madrid finals in 2011 and 2012.
 Elise Mertens vs. Elena Rybakina (0-1)
Last Meeting: Rybakina d. Mertens, 6-4, 4-6, 6-2, 2020 Shenzhen QF.
Rybakina is still searching for the sizzling form that kicked off the 2020 season, when she made the final or won the title at four of her first five events. Rybakina has not been the same consistent threat since the tour restarted last summer, but her best results after the break came on clay last fall, where she made the Strasbourg final and Round of 16 in Rome.
Already in the midst of a great 2021 season, Mertens has started her clay season well, making the final in Istanbul last week. The winner will play either Simona Halep or Zheng Saisai.
STAT: Of Rybakina's last 10 complete match losses, eight have gone three sets.
 Maria Sakkari vs. Anett Kontaveit (4-4)
Last Meeting: Kontaveit d. Sakkari, 2-6, 6-3, 11-9, 2021 Gipplsand Trophy SF.
Sunday will see the ninth meeting between good friends Sakkari and Kontaveit, an astounding tally given both are just 25 years old. Their last meeting was a dramatic affair, with Kontaveit saving match points and ultimately winning the match-tiebreak on a lucky netcord winner at the Grampians Trophy in Melbourne. The two have split their two meetings on clay, with their most recent coming at 2019 Rome. Sakkari won, 6-3, 6-2.
"We have had some close battles, and we played each other in Australia," Kontaveit said. "Yeah, it's going to be very tough. I mean, she's playing really good tennis. She did so well in Miami.
"When I'm out of the tournament, I always cheer for Maria. She's a very good player and it's going to be very tough. I think we always have competitive matches, which is nice."
STAT: Sakkari has struck 91 aces this season, the ninth most on tour.