Gauff showed plenty of resilience to come back from a 5-1 first-set deficit to win in a tiebreak. Despite dropping the second set, also in a tiebreak, the US Open champion cruised to a final set 6-2 victory to book her spot in the Australian Open semi-final against Aryna Sabalenka.
The battle between Gauff and Kostyuk lasted for over three hours, and the American was certainly not at her best as she failed to drop a set in her previous four Australian Open matches.
When looking back on Gauff's performance Arnaud Clement, who reached the 2001 Australian Open final, told Eurosport that Gauff failed to show much in terms of quality against Kostyuk.
"It was really very surprising. We are not going to put the two players in the same bag. Kostyuk took a lot of risks, she had success, she made a lot of unforced errors. Opposite, Coco Gauff did nothing the entire game.”
"She could have lost this meeting. And just when we tell ourselves that it’s okay, that she’s back from 1–5 to the first, that she’s going to relax, she tenses up again. At no time did she manage to free herself. Tennis-wise, it was extremely poor."
With world number two and reigning Australian Open champion Sabalenka up next, Gauff will need to be more assured and might be punished with a lower-level performance.
Clement raised concerns about the teenager's game and explained that she must offer some response to remain in contention for her first Australian Open triumph.
“Frankly, I have rarely commented on a match of this quality at this stage of the competition. We will need a very, very big reaction from Coco Gauff in the next match because if she plays like this, it will be very quick whoever her opponent is.”