Naomi Osaka overcomes wobble to beat Kostyuk

Naomi Osaka, the US Open's fourth seed, put in a spirited display to beat her teenage opponent Marta Kostyuk 6-3, 6-7(4), 6-2.

The Ukrainian received some treatment for an ankle injury in the second set, but still forced the former champion to a tie-break, having already dropped set point on her serve.

But she levelled up on the second set point in the breaker - and Osaka promptly sent her racket skidding across the court.

In the decider, the real difference between the two was Kostyuk's failure to take her many opportunities to break while racking up her unforced error count - 51 by the end of the match.

She converted just one of her eight break opportunities - and although Osaka was also wasteful, she did convert five of her 21.

Nevertheless, the former world number one was impressed by her young adversary.

"I think my biggest takeaway from her game is that she didn't back down from me or from, like, the speed of my ball, which for someone that young is kind of amazing, because I feel like the first set went that way, and then she could have faded in the second set," Osaka said to reporters afterwards.

"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."