Fourth seed Coco Gauff played nowhere near her best but still managed to scrape past Ukraine’s Marta Kostyuk and reach the Australian Open semi-finals.

The 19-year-old American made 51 unforced errors and hit just 17 winners in a 7-6 (8-6) 6-7 (4-7) 6-2 victory.

The US Open champion fought back from 5-1 down in the first set and could not serve out the second but kept calmer than Kostyuk in the decider.

Gauff will play Aryna Sabalenka or Barbora Krejcikova in the last four.

“I’m happy, really proud of the fight I showed,” said Gauff, who had never previously gone beyond the fourth round in Melbourne.

“Marta is a tough opponent and always makes it hard. I really fought and left it all on the court.”

Defending champion Sabalenka takes on Czech ninth seed Krejcikova in their quarter-final later on Tuesday.

The 25-year-old Belarusian, who lost to Gauff in the US Open final in September, is yet to drop a set at Melbourne Park this fortnight, conceding just 11 games on her way to the last eight.

The other two quarter-finals in a women’s singles draw blown open by a string of high-profile early exits take place on Wednesday.

One of China’s Zheng Qinwen, Russia’s Anna Kalinskaya, Ukraine’s Dayana Yastremska and Czech Republic’s Linda Noskova – none of who have ever played in a Grand Slam semi-final – will meet Gauff, Sabalenka or 2021 French Open champion Krejcikova in Saturday’s showpiece.

Aiming to become the first player since Naomi Osaka three years ago to win back-to-back majors, Gauff had breezed into the Australian Open last eight with a series of clinical and composed performances.

However, her display against world number 37 Kostyuk – the highest-ranked player she has faced so far – was far from convincing.

Both players looked tight and tense in a match where they made 107 unforced errors in a total of 246 points.

The three-hour contest featured 16 breaks of serve and Gauff took nine of them – including three in the decider.

The teenager has looked a different proposition over the past six months, playing with more confidence and trust in her ability under the guidance of renowned coach Brad Gilbert.

Showing more faith in her previously shaky forehand has been a key factor in her renaissance after a chastening first-round exit at Wimbledon last year.

Since then, Gauff has lost only four matches and remains unbeaten at the start of the 2024 season.

But that old failing reappeared, particularly in the second set, as Gauff had to dig deep to find a way past Kostyuk.

On her resilience, Gauff said: “I was just trying to get one more game in the first set, make it competitive and then one game turned into another.

“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.”

Kostyuk, 21, was playing in her first major quarter-final and says she is “proud” of a run she hopes will remind people of the ongoing Russian invasion of Ukraine.

“I was texting with some people from [Ukrainian capital city] Kyiv. I said ‘How is it? How are you guys?’ They said ‘Well, we were looking between your score and where the missiles are flying,” Kostyuk said.

“It’s very exhausting to live in this state. The war is still there. It’s not gone.”