Coco survives Kostyuk test to reach Australian Open semis

US Open champion Coco Gauff came through a huge test at the hands of Ukrainian world number 37 Marta Kostyuk 7-6(6) 6-7(3) 6-2 to reach the semi-finals of the Australian Open for the first time on Tuesday.

The fourth-seeded American will rarely play as badly and still progress but prevailed after more than three hours on a blistering hot Rod Laver Arena to fulfill her coach Brad Gilbert's famous maxim by "Winning Ugly".

"I'm really proud of the fight I showed today, Marta's a tough opponent, every time we play it's a tough match," she said.

"Yeah, I really fought and left it all out on the court today."

Gauff, playing her final Grand Slam as a teenager, had swept into the last eight on a nine-match winning streak as she looked to back up her first major success in New York last year.

Kostyuk got right in the American's face from the off, however, and stormed to a 5-1 lead, serving twice for the opening set as well as holding a set point on Gauff's serve.

The 19-year-old American knew she was up against it and, problem-solving on her feet, battled her way back into the contest by rattling off five successive games to serve for the set herself.

"I was playing not great. I was just missing everything on both wings and not serving well. I was just trying to win one extra game," Gauff recalled.

"I believe every point, every game matters, and eventually the score started to get closer."

Kostyuk had been looking increasingly frequently at her coach as her confidence waned but she earned three break points off Gauff's forehand and forced the tiebreak when the American double-faulted.

The Ukrainian was revived by treatment on blisters on her feet before the tiebreak but blew her second set point, allowing Gauff to come racing into the net to go a set up.

The players traded breaks throughout a second set featuring some lengthy rallies but it was Gauff who was able to edge ahead and serve for the match at 5-3.

Again, however, a combination of Gauff's frail second serve and Kostyuk's ability to conjure up winners - she fired 39 across the contest - allowed the Ukrainian to get back on serve and then level up the contest at one-set all.

Gauff found the fix as Kostyuk tired in the third set by ramping up the pace of her first serve and backhand but was broken when serving for the match for the second time.

She finally got over the line at the third time of asking to move into a semi-final against either Aryna Sabalenka - a rematch of last year's Flushing Meadows final - or Czech ninth seed Barbora Krejcikova.

Kostyuk, who was playing in her first Grand Slam quarter-final, was remarkably upbeat despite the defeat.

"For me, it's a win because I was playing one of the best girls in the world. Managed to be still very close," the 21-year-old said.

"It feels far but also very close. This whole tournament I think is a big win for me."