With shadows creeping across the Arthur Ashe Stadium court in the early afternoon, the 137th-ranked Kostyuk certainly had her chances to pull off a significant surprise, too.
The key moment: Kostyuk held five break points that could have given her a 3-1 lead in the final set.
But Osaka fended off every one and held to 2-all, beginning her match-closing run.
“I’m kind of scared how she’s going to be in the future,” said Osaka, who played with tape wrapping her left hamstring, which has been a problem since last week. “She has no fear.”
Kostyuk had her own issues: She twice took a medical timeout to have a trainer tape her right ankle.
But she also was able to control the outcomes of points for stretches, winning 19 of the 23 points when she went to the net. The Ukrainian teen also delivered more winners than Osaka, 36-30, over the match’s 2 1/2 hours.
“For me, I just feel like I had so many points that I didn’t capitalize on,” Osaka said, acknowledging she was “too passive” sometimes.
No. 8-seeded Petra Martic advanced to the fourth round for the second straight year with a 6-3, 6-3 victory over Varvara Gracheva.
Gracheva made too many mistakes, with 33 unforced errors — three times as many as Martic.
As Gracheva’s deficit grew larger and larger, she knew more than anyone that a comeback was still possible. In the previous round, she trailed No. 30 Kristina Mladenovic 6-1, 5-1 and faced four match points before coming all the way back to win.
Petra Martic of Croatia returns a shot to Varvara Gracheva of Russia during the third round of the US Open. Martic won in straight sets. AP Photo/Seth Wenig
There was no such turnaround this time.
Martic now will try to get to the second Grand Slam quarterfinal of her uncareer after reaching that stage at last year’s French Open.
She has lost her other six fourth-round matches at major tournaments, including against Serena Williams at the US Open last year.
Also moving into the fourth round: 2016 U.S. Open champion Angelique Kerber, who beat 20-year-old American Ann Li 6-3, 6-4.
Kerber’s next opponent is another American, 28th-seeded Jennifer Brady, a 6-3, 6-3 winner against Caroline Garcia.