NEW YORK -- Seemingly back in her US Open match, suddenly a point from getting to a third set, Naomi Osaka lost her way Thursday night, missing forehand after forehand until she ceded that game and chucked her racket, sending it clattering on the Arthur Ashe Stadium court.

Not much later, she was out of the tournament, eliminated 6-3, 7-6 (5) by Karolina Muchova in the second round of an event where Osaka won two of her four Grand Slam titles.

Osaka -- once ranked No. 1 but now No. 88 after missing nearly 1 years because of mental health breaks and time off to have a baby -- struggled for much of the early going, dropping five consecutive games and 22 of 26 points in one stretch.

But she played much better in the second set, getting her only break of the match to lead 5-4 and yelling "Come on!" when Muchova netted a forehand. The crowd roared for Osaka.

Serving for that set, Osaka 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's when Osaka really faltered, making five forehand errors, with a double-fault mixed in, to waste all three of those set points and, worse, get broken.

When they got to the tiebreaker, it was Muchova who asserted herself, then used some scrambling defense on the last point, flinging the ball back over the net and seeing Osaka send a swinging volley out.

"This is unbelievable -- the atmosphere and the people. This is crazy energy," said Muchova, a 28-year-old from the Czech Republic.

She enjoyed a breakout season in 2023, reaching the final at the French Open before losing to Iga Swiatek, and the semifinals at the US Open before exiting against eventual champion Coco Gauff. But shortly after that run at Flushing Meadows, Muchova left the tour because of an injured right wrist, and she had surgery in October.

She was sidelined until this June; her Grand Slam return was a first-round loss at Wimbledon last month.

"Honestly, this year, the biggest win for me is that I could play again," Muchova said. "This is just a cherry on top, to be here again, in this stadium."

On this brisk evening, with the temperature dipping to 70 degrees after topping 90 on Wednesday afternoon, Muchova did not look at all like someone who is currently ranked 52nd.

Using a pen to jot down thoughts in a notebook during changeovers, Osaka was never able to seize control of the on-court exchanges.

Her groundstrokes were not as perfect as they were during a 6-3, 6-2 victory over 2017 French Open champion Jelena Ostapenko on Tuesday. Osaka did not make a single unforced error until the second set of that one, which became her first win against an opponent ranked in the top 10 in more than four years.

But if Osaka played so-so this time, Muchova was the one who looked terrific, whether serve-and-volleying or mixing in slices, finding her spots with serves or turning up the power when she wanted. Muchova serve-and-volleyed seven times in the match, winning six of those points. She has serve-and-volleyed 20 times for the tournament, easily the most of any woman (next closest was Japanese qualifier Ena Shibahara with 5).

From the moment Osaka went ahead 3-2 at the start, everything went in Muchova's direction through the end of that set. And just as it seemed Osaka was getting back into the contest -- with thousands of spectators supporting her -- her forehand let her down.

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