Like Osaka, Sabalenka has easy power in abundance and a potent first serve along with a more vicious second serve. Unlike Osaka, Sabalenka has a face that is often an open book, and there was a wide range of feelings on display Monday, including disgust as she double-faulted on match point and reacted by hurling her racket at her chair.

It was an uninspiring finish to a match that was brimming with thrills.

Osaka was the steadier flame early before Sabalenka burned brighter to dominate the second set. Osaka did her best to change the momentum and the mood, asking for the 10-minute heat break permitted by the rules before the third set. She left the court in Louis Armstrong Stadium while Sabalenka stayed in her chair, putting ice towels on her neck and legs while watching the entertainment on the new stadium scoreboard.

Others might have lost their focus or rhythm, but Sabalenka jumped out to a 2-1 lead when play resumed, breaking Osaka in the third game as she stuck to her plan of attacking second serves.

But with the match under control, Sabalenka lost her grip in the next game, double-faulting at 30-15 and then missing a low forehand to allow Osaka to break back. Though Sabalenka would later show plenty of moxie to come back from 0-40 to hold serve in the sixth game, she could not find a way to threaten Osaka’s serve again.

“I was thinking a lot about the future, but I didn’t stay in the moment,” Sabalenka said. “I think she was in the moment, and she was just trying to play each point, win each point, and I think this was the key for her. Next time I’m going to try to learn in the important moments to stay in the moment, but not think about future, past or something else.”

It was a big afternoon for Japanese tennis in Armstrong, with Kei Nishikori reaching the men’s quarterfinals by defeating Philipp Kohlschreiber, 6-3, 6-2, 7-5, in the opening match.

This is the first time a Japanese man and woman have reached the final eight in singles at the U.S. Open. The only previous time it happened at a Grand Slam tournament was in 1995 at Wimbledon, where Kimiko Date reached the women’s quarterfinals and Shuzo Matsuoka reached the men’s quarterfinals.