NEW YORK, NY, USA - No.4 seed Naomi Osaka passed a stern test against resurgent teenager Marta Kostyuk to reach the fourth round of the US Open for the third year in a row, surviving a series of wild momentum shifts to emerge a 6-3, 6-7(4), 6-2 victor after two hours and 33 minutes.

In what turned into a riveting contest, the 2018 champion was forced to summon all of her resilience to overcome her opponent's hustling footspeed, ability to ratchet up her competitive intensity and boldness in essaying 23 net approaches. Ultimately, Osaka would run away with the final set, having escaped four points to fall behind a break in it - but not before the 18-year-old Kostyuk had made a serious impression on both her and anyone watching. "I'm kind of scared of good she's going to be in the future," Osaka - who wore a mask honoring Ahmaud Arbery today - admitted in her on-court interview.

Afterwards, she expounded: "My biggest takeaway from her game is that she didn't back down from me or from the speed of my ball, which for someone that young is kind of amazing... The first set went [my] way, and then she could have faded in the second set. Of course in the third set towards the end I feel like physically I was better, and I was, of course, more experienced. But, yeah, I feel like she's definitely going to be dangerous."

Nonetheless, 30 winners to 38 unforced errors - compared to Kostyuk's 36 winners and 51 unforced errors - and an 81% winning percentage behind her first serve enabled Osaka to maintain her dominant recent record against players younger than herself. Since the start of her breakthrough year of 2018, she has now won 12 out of 14 such matchups, with her only losses coming to Sofia Kenin at Cincinnati 2019 and Coco Gauff at the Australian Open this January.

Like Gauff, Kostyuk is a player who seized mainstream attention on the Grand Slam stage at the age of 15, when as a qualifying wildcard ranked World No.521 she reached the third round of the 2018 Australian Open. The Ukrainian did not surge on to the Tour in the wake of that - by April 2019, her ranking had fallen to World No.320, and that summer she told WTA Insider: "I was the first who broke through and the first who fell."

Still only 18, Kostyuk's comeback has come in the form of hard graft on the ITF Pro Circuit - but, having reached the round of 32 twice in as many Grand Slam main draws, was ready to show off the fruits of her improvement.

Showing off fluid defensive skills as well as the ability to hang with Osaka's pace in the opening stages, Kostyuk particularly impressed with her ability to nail brilliant passes at full stretch, while a point where she found herself dragged unexpectedly into net turned into a showcase of her hand skills, reflexing a winning volley off a full-power Osaka rocket. The World No.137 was also fearless when against the wall, coming up with her boldest and most aggressive plays in overturning triple break point to fall behind a double break.

But Osaka, as though rectifying her performance against Gauff in Melbourne, remained cool-headed and in control of her own side of the court. Winning 86% of her first serve points in the first set compared to Kostyuk's 46%, the Japanese No.1 did not face a break point in the opening act. When Kostyuk opened the door courtesy of two double faults in the fifth game, Osaka kicked it in to break with a superb running forehand. Four games later, another huge forehand down the line sealed her third set point with another break of Kostyuk's serve.

But eventually, Kostyuk's ability to suddenly raise her level when down break point paid off. The 2017 Australian Open junior champion would save 16 out of 21 in total, including seven in her first two service games of the second set. Six of those came in a marathon fourth game, and two were saved with clean aces en route to a hard-fought Kostyuk hold. As though liberated by passing those tests, the youngster suddenly found her groove on serve to find her most dominant holds of the day - and on return.

A clean backhand return winner in the seventh game paved the way for a first break of the Osaka serve - and having seized the momentum, Kostyuk found herself a game away from levelling the match.

Now, it was Osaka's turn to be clutch. Serving to stay in the set, the former World No.1 needed to survive an edge-of-seat tussle to fend off Kostyuk, whose play oscillated from irresistible to wild. A service winner saved a set point; three deuces - and one brilliant Osaka defensive rally - later, the 22-year-old was temporarily out of danger thanks to a netted Kostyuk forehand.

Having come out of the wrong end of such an intense game, it wasn't entirely surprising that Kostyuk was unable to serve the set out - but showing even more resilience, the Kyiv native held on from 5-6 down to force a tiebreak. Therein, from 0-2 down she ran off seven out of nine points to steal a rollercoaster of a set. The exchange with which Kostyuk brought up her second set point was arguably emblematic: having got herself in trouble after essaying a poor dropshot, she was nonetheless able to reflex a winning volley off Osaka's putative full-blast pass.

"I was very displeased with how I was performing towards the end of the second set," recalled Osaka. "The thing that made me most displeased was probably the decisions that I was making and the fact that I started becoming way too passive and hoping that she would make an unforced error. For me, I felt everything was going really well for her during the tiebreak. And I remember she came to the net, and I hit it as hard as I could towards her, and she made it, and for me I was just, like, Whoa, this is unbelievable."

With all the momentum, Kostyuk found herself in the driver's seat for the first time today - and as Osaka gifted her a double fault and pair of errors, held four break points to move up 3-1 in another of the match's crucial games. Consecutive return winners off each wing put Kostyuk on the verge of seizing it - but strong serving on each of her opportunities enabled Osaka to escape and, importantly, to regain the momentum.

"I feel like that game was very important," said Osaka afterwards. "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."

Having been unable to take those chances, Kostyuk wobbled in the next game, committing her eighth and ninth double faults to fall behind triple break point. Unlike Osaka, she could not wriggle out of it this time, with the two-time Grand Slam champion slamming a forehand crosscourt to take a 3-2 lead.

Having spent the past two hours turning the match into a scrappy feast of constantly shifting advantages, that pair of games would prove to be Kostyuk's last stand. Having pushed Osaka hard in so many marathon passages of play and survived so many twists and turns against her, the last four games passed by in a flash. Osaka was fully dominant on serve as Kostyuk lost control of her groundstrokes, and the World No.9 reeled off 16 of the last 19 points to set up a fourth-round clash against either No.14 seed Anett Kontaveit or No.24 seed Magda Linette.

2020 US Open highlights: Osaka survives Kostyuk