MELBOURNE, Australia (AFP) — US Open champion Coco Gauff gave herself only a "C" rating for an error-strewn performance that put her into a first Australian Open semifinal Tuesday, but said she hoped it was a one-off.

The American world No. 4 needed three hours and eight minutes to wear down Ukraine's unseeded Marta Kostyuk in stifling conditions on Rod Laver Arena 7-6 (8/6), 6-7 (3/7), 6-2.

She will face either defending champion and second seed Aryna Sabalenka or ninth-seeded Barbora Krejcikova on Thursday for a place in the final.

After showing ominous form to race into the last eight, Gauff admitted she had an off-day — she struggled on serve and fluffed countless opportunities, hitting just 17 winners.

"I think today was definitely a 'C' game, so didn't play my best tennis but really proud that I was able to get through today's match," said the 19-year-old, who had never been past the fourth round at Melbourne Park in four previous attempts.

"Hopefully, got the bad match out of the way and I can play even better."

Both players were responsible for the low-quality contest, with an incredible 16 service breaks throughout the match, and the pair racked up 107 unforced errors between them.

Kostyuk will rue missing her chances, letting slip a 5-1 lead and squandering two set points in the first set.

Gauff also faltered at crunch moments, broken when serving for the match in the ninth game of the second set before keeping her cool as Kostyuk became more agitated in the deciding set.

"I was just trying to get one more game in the first set and at least make it competitive and then one game turned to another and I was able to win that set," the American said.

"In the second, I gave myself opportunities but became a little bit passive and when I came out in the third I was just trying to play aggressive and hit through the court."

She credited what she had learned in winning the US Open last year with helping her "problem solve" in key moments.