NEW YORK — Naomi Osaka seemed to be back in her US Open match and was suddenly one point away from the third set. But on Thursday night, she lost her way, missing one forehand after another until she gave up the game and threw her racket in the air, sending it crashing onto the court at Arthur Ashe Stadium.

Shortly afterwards she was out of the tournament, eliminated 6-3, 7-6 (5) by Karolina Muchova in the second round of an event in which Osaka won two of her four Grand Slam singles titles.

Osaka, once ranked No. 1 but now No. 88 after a nearly 1½-year absence due to mental health issues and maternity leave, admitted it was “a little bit difficult” to withdraw in the second round.

“It’s been a little bit difficult because of course I can only go by the results,” Osaka said. “I feel faster. I feel better, but I lost in the second round. … Yeah, it’s a little bit difficult because I take these losses really personally. It’s a dramatic word, but I feel like my heart dies every time I lose. It’s very bad, but I’ve tried to be more mature and learn more about it and talk about it.”

Osaka struggled early on, losing five games in a row and 22 of 26 points in one period.

But she played much better in the second set, taking her only break of the match to lead 5-4 and shouting “Come on!” when Muchova hit a forehand. The crowd cheered for Osaka.

Osaka served for that set and hit a 119 mph ace, her fastest of the match, to lead 40-love. That gave her three chances to extend the match to a third set. That was when Osaka really faltered, making five forehand errors, with a double fault in between, to waste all three of those set points and, worse, get broken.

When it came time for the tiebreak, it was Muchova who asserted herself. She defended on the last point and threw the ball back over the net. Osaka volleyed the ball out.

“This is incredible — the atmosphere and the people. This is crazy energy,” said Muchova, a 28-year-old from the Czech Republic.

She had a breakthrough season in 2023, reaching the final at the French Open before losing to Iga Swiatek, and the semifinals at the US Open before falling to eventual champion Coco Gauff. But shortly after that run at Flushing Meadows, Muchova left the tour with an injured right wrist, and underwent surgery in October.

She was sidelined until June; her return to a Grand Slam tournament last month came with a first-round defeat at Wimbledon.

“Honestly, the biggest victory for me this year is that I was able to play again,” Muchova said. “This is just the icing on the cake, to be here again, in this stadium.”

On this chilly evening, with temperatures dropping to 21 degrees Celsius after reaching 32 degrees Celsius on Wednesday afternoon, Muchova looked nothing like someone currently ranked 52nd.

Osaka was never able to gain control over the exchanges on the field, as he would jot down his thoughts in a notebook with a pen during player substitutions.

Her groundstrokes weren’t as perfect as they were during a 6-3, 6-2 victory over 2017 French Open champion Jelena Ostapenko on Tuesday. Osaka didn’t make a single unforced error until the second set of that set, her first victory against a top-10 opponent in more than four years.

But if Osaka was so-so this time around, it was Muchova who looked great, whether she was serving and volleying or mixing slices, finding her spots on serve or pushing the power when she wanted to. Muchova served and volleyed seven times in the match, winning six of those points. She served and volleyed 20 times for the tournament, by far the most of any woman (the closest was Japanese qualifier Ena Shibahara with five).

From the moment Osaka led 3-2 at the start, everything went Muchova’s way until the end of that set. And just when it looked like Osaka was getting back into the match — with thousands of spectators rooting for her — her forehand let her down.

“I felt like I had to keep fighting,” Osaka said. “I didn’t feel like I was playing as perfect as before, but I don’t think you can play every match perfectly. … I think during the stressful moments I got nervous, and I don’t know if I should just keep playing more matches and get used to that feeling, especially on a really big stage.

“Honestly, if I get over the disappointment, I’m quite proud of myself that I was given so many chances and still feel like I could have played much better.”

ESPN Stats & Information and The Associated Press contributed to this report.