Naomi Osaka, trailing by a set, squandered three sets points that would have leveled her match and crashed out of the U.S. Open in the second round on Thursday in New York.

The Czech Republic's Karolina Muchova beat Osaka, a two-time U.S. Open champion, 6-3, 7-6 (5).

In terms of rankings, the match doesn't qualify as an upset, as Muchova is No. 52 in the world and Osaka is No. 88. But Osaka is a former world No. 1 and a four-time Grand Slam winner. Her ranking slipped during her year-plus absence from the game due to maternity leave and a mental-health break.

Osaka, 26, broke serve to take a 5-4 lead in the second set, and she went up 0-40 in the next game. Muchova fought back to hold serve and extend the set.

In the tiebreaker, Osaka jumped in front 4-2, but Muchova captured the next four points. Muchova closed it out on her second match point.

"I think during the pressure moments I got nervous, and I don't know if I just have to keep playing more matches and get used to that feeling, especially on a really big stage," Osaka said. "Honestly, if I get past the disappointment, I feel pretty proud of myself to have gotten that many opportunities while still feeling like I could have played much better."

The result ends a disappointing year in the majors for Osaka, who lost in the first round at the Australian Open and in the second round at the other three: the French Open, Wimbledon and the U.S. Open.

A wrist injury kept Muchova, 28, out of the season's first two majors, and she lost in the opening round at Wimbledon. She enjoyed Grand Slam success last year, when she got to the French Open final and reached the U.S. Open semifinals.

"Honestly, this year the biggest win for me is that I could play again," Muchova said. "I'm really grateful I'm able to play tennis and that I made it here. This is just the cherry on top to be here in this stadium. It's crazy."

Earlier Thursday, world No. 1 Iga Swiatek of Poland nearly delivered her second-round opponent a double bagel, instead settling for a 6-0, 6-1 triumph over Japanese qualifier Ena Shibahara.

The match lasted just 65 minutes, though much of that was eaten up by the second game of the second set, where Shibahara rallied from a 40-15 deficit to win a game that featured eight deuces. That got Shibahara on the board, but it was all Swiatek the rest of the way.

Swiatek's victory was much more dominant than her first-round trial against Kamilla Rakhimova of Russia, which ended 6-4, 7-6 (6).

"I wasn't really focusing on a score," Swiatek said post-match. "I just wanted to focus on technicalities and what I practiced yesterday and trying to convert it to my match. I was just trying to stay in the zone. It didn't matter to me if it was 3-all or 6-1. I just want to keep playing the same way."

Swiatek's serve overpowered Shibahara. The top seed won 20 of 24 points on her first serve (83.3 percent) as well as 8 of 10 on her second serve (80 percent). She never faced break point and had just six unforced errors to Shibahara's 28.

The big news of the afternoon was Kazakh fourth seed Elena Rybakina withdrawing from the major due to injuries. France's Jessika Ponchet advanced to the third round via walkover.

"Unfortunately, I have to withdraw from my match today due to my injuries," Rybakina said in a statement. "I did not want to finish the last Grand Slam of the year this way, but I have to listen to my body, and I hope I can close out the remainder of the year strong."

No. 5 seed Jasmine Paolini of Italy moved on to the next round when Czech opponent Karolina Pliskova retired due to injury three points into the match.

No. 6 seed Jessica Pegula defeated fellow American Sofia Kenin 7-6 (4), 6-3 to push ahead into the Round of 32. Pegula, who has had a red-hot summer with a WTA 1000 title in Toronto followed by an appearance in the Cincinnati final, has never made it to the semifinal of any Grand Slam event.

Pegula's Round of 32 opponent will be Spaniard Jessica Bouzas Maneiro, who took out No. 31 seed Katie Boulter of Great Britain 7-5, 7-5.

Ashlyn Krueger of the U.S. upset No. 21 seed Mirra Andreeva of Russia 6-1, 6-4. Krueger, a 20-year-old from Missouri, is on to the third round after never making it past the first round at a major.

Other winners Thursday included No. 15 seed Anna Kalinskaya, No. 16 Liudmila Samsonova, No. 18 Diana Shnaider and No. 25 Anastasia Pavlyuchenkova, all of Russia; No. 22 Beatriz Haddad Maia of Brazil; No. 30 Yulia Putintseva of Kazakhstan; Sara Errani of Italy; and Anastasia Potapova of Russia.

In the last match of the night, Denmark's Caroline Wozniacki powered past Mexico's Renata Zarazua 6-3, 6-3.

--Field Level Media