Il Nuovo Postiglione - Gauff gives herself 'C' rating after lacklustre march into Australian Open semis

The American world number four 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," said the American.

"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.

"It was really important. Today was frustrating because I knew how I needed to play, I just couldn't execute," said Gauff, who is yet to meet a seed after a string of top names were knocked out early.

"But, yeah, problem-solving is tennis. You know, you could play someone today and maybe a week later they'll play you completely different.

"It's a part of it. At the end of the day you're given a scouting report but you have to go with feeling and try your best to win. That's what I did today."

The win extended Gauff's unbeaten streak this year to 10 matches after she won at Auckland in the lead-up.

Kostyuk, ranked 37, said he was "very proud" despite the defeat after her deepest run at a Grand Slam.

"I won for myself today, and I think it's the most important thing," said the 21-year-old.

F.Criscuolo--INP