"She's very good, I'm kind of scared of how she's going to be in the future," Osaka said of Kostyuk after the match.
In a night-session feature match, Djokovic posted his second victory in as many weeks over Germany’s Struff to storm into the last 16, where he faces off with Spanish No.20 seed Pablo Carreno Busta.
The world number one extended his unbeaten record this season to 26-0 with a 6-3, 6-3, 6-1 success over Struff, who also lost to the Serb in last week's Western & Southern Open, and in January's Australian Open.
Meanwhile, Angelique Kerber, the 2016 champion, strolled into the fourth round without dropping a set, dismissing American world number 128 Ann Li 6-3, 6-4 to book a showdown with in-form 28th seed Jennifer Brady.
Canadian No.12 seed Denis Shapovalov came back from 2-5 down in the fourth set en route to a battling 3-6, 6-3, 4-6, 7-6 (7-5), 6-2 victory over American Taylor Fritz.
It is Shapovalov’s second trip to the US Open fourth round from four tournament appearances and he’ll be looking to make his first grand slam quarter-final when he takes on Belgian seventh seed David Goffin on Sunday.
After two four-setters in his opening two rounds, Goffin punched his ticket to the last 16 in straight sets, 6-1, 7-6 (7-5), 6-4 over Serbian 26th seed Filip Krajinovic.
Earlier on Arthur Ashe stadium, Kostyuk produced some incredible tennis to stretch Osaka to three sets and will re-enter the top 120 when the new rankings are released after the US Open.
The world number 137 from Kiev enjoyed a breakthrough run to the third round as a 15-year-old qualifier at the Australian Open in 2018 but has struggled to follow that up with big performances since.
A crucial fourth game in the deciding set saw Osaka save five break points to hold serve, and that proved to be the turning point for the world No.10, who swept the next four games to book a last-16 meeting with Anett Kontaveit.
"I feel like that [fourth] game was very important, and honestly, I'm not sure how it would have went if she won it and I would have had to break her back, but thankfully I don't have to think about it too much," said the 22-year-old.
Osaka, the 2018 champion, has worn a different mask, emblazoned with the name of a different police shooting victim, to each of her matches so far this week.
For her third round, she donned a mask with the name Ahmaud Arbery, the black man who was shot dead as he went for a jog in a Georgia town in February.
Osaka will be facing the 14th-seeded Kontaveit for the second time in as many weeks, having defeated her at the Western & Southern Open quarter-finals in three sets.
Two-time Wimbledon champion Petra Kvitova earned her place in the last 16 with a 6-4, 6-3 result against American Jessica Pegula. Her next test comes in the form of Shelby Rogers, who was a 6-2, 6-4 victor over Madison Brengle.
“I know Shelby played great in Lexington. She has a great run here so far, as well. She worked hard probably in the lockdown. I know she had some injuries, so it's really great to see her back playing great,” said Kvitova.
In a night-session feature match, Djokovic posted his second victory in as many weeks over Germany’s Struff to storm into the last 16, where he faces off with Spanish No.20 seed Pablo Carreno Busta.
The world number one extended his unbeaten record this season to 26-0 with a 6-3, 6-3, 6-1 success over Struff, who also lost to the Serb in last week's Western & Southern Open, and in January's Australian Open.
Meanwhile, Angelique Kerber, the 2016 champion, strolled into the fourth round without dropping a set, dismissing American world number 128 Ann Li 6-3, 6-4 to book a showdown with in-form 28th seed Jennifer Brady.
Canadian No.12 seed Denis Shapovalov came back from 2-5 down in the fourth set en route to a battling 3-6, 6-3, 4-6, 7-6 (7-5), 6-2 victory over American Taylor Fritz.
It is Shapovalov’s second trip to the US Open fourth round from four tournament appearances and he’ll be looking to make his first grand slam quarter-final when he takes on Belgian seventh seed David Goffin on Sunday.
After two four-setters in his opening two rounds, Goffin punched his ticket to the last 16 in straight sets, 6-1, 7-6 (7-5), 6-4 over Serbian 26th seed Filip Krajinovic.
Earlier on Arthur Ashe stadium, Kostyuk produced some incredible tennis to stretch Osaka to three sets and will re-enter the top 120 when the new rankings are released after the US Open.
The world number 137 from Kiev enjoyed a breakthrough run to the third round as a 15-year-old qualifier at the Australian Open in 2018 but has struggled to follow that up with big performances since.
A crucial fourth game in the deciding set saw Osaka save five break points to hold serve, and that proved to be the turning point for the world No.10, who swept the next four games to book a last-16 meeting with Anett Kontaveit.
"I feel like that [fourth] game was very important, and honestly, I'm not sure how it would have went if she won it and I would have had to break her back, but thankfully I don't have to think about it too much," said the 22-year-old.
Osaka, the 2018 champion, has worn a different mask, emblazoned with the name of a different police shooting victim, to each of her matches so far this week.
For her third round, she donned a mask with the name Ahmaud Arbery, the black man who was shot dead as he went for a jog in a Georgia town in February.
Osaka will be facing the 14th-seeded Kontaveit for the second time in as many weeks, having defeated her at the Western & Southern Open quarter-finals in three sets.
Two-time Wimbledon champion Petra Kvitova earned her place in the last 16 with a 6-4, 6-3 result against American Jessica Pegula. Her next test comes in the form of Shelby Rogers, who was a 6-2, 6-4 victor over Madison Brengle.
“I know Shelby played great in Lexington. She has a great run here so far, as well. She worked hard probably in the lockdown. I know she had some injuries, so it's really great to see her back playing great,” said Kvitova.