Earlier, Gauff had proven all of her battling qualities as she fought by damage to beat Kostyuk.
The American obtained therapy on her decrease again halfway by the match and shortly discovered herself 5-1 down within the second set.
Nonetheless, the world quantity 4 rallied and reeled off six video games in a row to finish a 6-4 7-5 win.
“Bodily, I wasn’t at my finest, which I am probably not used to,” Gauff advised Sky Sports activities.
“It was like a psychological problem, studying find out how to play by that.”
The lengthy tennis season has taken a toll on a lot of gamers, with Ukraine’s Kostyuk additionally receiving therapy on a wrist damage that has been hampering her since August.
It’s the second time that Gauff has overturned a 5-1 deficit at this event, having completed the identical to beat Pegula on Sunday.
Nonetheless, the 22-year-old admitted she was not anticipating it to occur once more, and was already getting ready herself for a 3rd set.
“I requested my physio at 5-2 down, what can I do on the lavatory break [between sets] to attempt to loosen up,” Gauff stated.
“After which there was no rest room break!”
Gauff struggled along with her serve all through, producing 9 double faults to 1 ace, however stated afterwards she struggled to get by her movement with out feeling ache.
Kostyuk – a late call-up to the season-ending finals after Elena Rybakina’s withdrawal – additionally struggled to generate tempo and spin due to her wrist drawback.
World quantity 4 Gauff got here again from an early break all the way down to take the primary set, however instantly bumped into issues within the second.
She referred to as for the coach after going 3-0 down however quickly trailed by a double break, with Kostyuk saving three break factors inbetween.
Nonetheless, along with her motion hampered and the gang behind her, Gauff was in a position to swing arduous and in the end outlast Kostyuk.
