Naomi Osaka suffered the biggest shock defeat to date at the 2021 Western and Southern Open on Thursday.
The No. 2 seed in the women's singles bracket fell to unseeded Jil Teichmann in the round of 16 in Cincinnati.
The second-ranked player in the men's singles also experienced some difficulties, but Stefanos Tsitsipas was able to rally for a three-set victory over Lorenzo Sonego.
The top seeds in each bracket had much easier times on the hard court. Daniil Medvedev and Ashleigh Barty both breezed through their respective matches to reach the quarterfinals.
Barty, Medvedev and Tsitsipas will all play on Center Court on Friday. The full order of play can be found here.
No. 1 Daniil Medvedev def. Grigor Dimitrov, 6-3, 6-3
No. 2 Stefanos Tsitsipas def. Lorenzo Sonego, 5-7, 6-3, 6-4
No. 3 Alexander Zverev def. Guido Pella, 6-2, 6-3
No. 4 Andrey Rublev def. Gael Monfils, 7-6 (7-2), 7-6 (7-5)
No. 12 Felix Auger-Aliassime def. No. 5 Matteo Berrettini, 6-4, 6-3
No. 7 Pablo Carreno Busta def. No. 9 Hubert Hurkacz, 7-6 (8-6), 7-6 (7-3)
No. 8 Casper Ruud def. No. 10 Diego Schwartzman, 6-4, 6-3
Benoit Paire def. John Isner, 7-6 (7-1), 6-7 (2-7), 6-1
Tsitsipas faced an early hole against Sonego.
The unseeded Italian hit 15 winners in the opening set and won the first three points in the opening game of the second set.
Then Tsitsipas responded and started to gain momentum in the match. He described his service hold at the beginning of the second set as his "game-changer," per ATPTour.com.
"After finding my game from that part of the match onwards, things started working pretty well for me," the 23-year-old said. "The psychology kind of changed and I was having that fighting spirit and not letting go was very crucial."
The Greek earned a crucial service break at the start of the third set as well to avoid an early elimination.
Tsitsipas will face off with Felix Auger-Aliassime in the quarterfinals. The 12th-seeded Canadian breezed past Wimbledon finalist Matteo Berrettini in straight sets.
They are two of seven seeded men remaining in the men's singles draw. Benoit Paire is the lone unseeded player left.
The Frenchman saw off the big-serving John Isner in the third set of their match to set up a clash with Andrey Rublev.
Medvedev will face Pablo Carreno Busta, and Casper Ruud goes head-to-head with Alexander Zverev.
The No. 1 seed has looked like the best player on the men's side through two matches, having won all four of his sets by three games or more.
No. 1 Ashleigh Barty def. No. 14 Victoria Azarenka, 6-0, 6-2
Jill Teichmann def. No. 2 Naomi Osaka, 3-6, 6-3, 6-3
No. 5 Karolina Pliskova def. Jessica Pegula, 6-4, 7-6 (7-5)
No. 9 Barbora Krejcikova def. No. 8 Garbine Muguruza, 6-1, 6-7 (5-7), 6-2
No. 10 Belinda Bencic def. Karolina Muchova, 7-5, 2-1 (retired)
No. 11 Petra Kvitova def. Ons Jabeur, 6-1, 6-2
Angelique Kerber def. Jelena Ostapenko, 4-6, 6-2, 7-5
Paula Badosa def. Elena Rybakina, 6-2, 7-6 (7-5)
Osaka looked solid in her opening set, but she failed to replicate that quality in the second and third sets.
The No. 2 seed hurt herself with too many unforced errors, which led to a 3-6, 6-3, 6-3 defeat at the hands of the 76th-ranked player in the world.
Teichmann said after the match that she tried to stick what she does best to take down one of the best players in the world, per WTATennis.com.
"I knew she's a very tough opponent. She's a Grand Slam champion. I'm the underdog, so... my plan was to just play my game," the 24-year-old said. "I move very good, I change up everything I can - directions, height, everything - and I think she didn't really feel very comfortable about it, and I served very good."
Teichmann is one of three unseeded women who advanced to the last eight, but she has the least decorated resume of that group. Angelique Kerber is a previous Grand Slam champion, and Paula Badosa reached the quarterfinals of the French Open.
Teichmann will take on her fellow Swiss player, Belinda Bencic, in the quarterfinals. Bencic won the night's final match after Karolina Muchova retired because of an injury.
Most of the seeded players who advanced to the last eight had relatively comfortable days on the court. Barty, Karolina Pliskova and Petra Kvitova all won in straight sets.
The Australian's win was the most impressive of the bunch because it came in a showdown with Victoria Azarenka, who made the U.S. Open final last year.
Barty is one of four major winners left in her side of the draw. She will face 2021 French Open champion Barbora Krejcikova, while Kerber and Kvitova go head-to-head.