Coco Gauff will face defending champion Aryna Sabalenka in her first Australian Open semi-final after coming through a huge test against Ukrainian Marta Kostyuk.
The fourth-seeded American, who won the US Open last September for her maiden Grand Slam title, prevailed 7-6 (8-6) 6-7 (3-7) 6-2 against the world No 37.
The 19-year-old will rarely play as badly and still progress but prevailed after more than three hours on a blistering hot Rod Laver Arena to fulfil 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," Gauff said.
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"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. But Kostyuk got right in the American's face from the off, storming into a 5-1 lead, serving twice for the opening set as well holding a set point on Gauff's serve.
Gauff 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 just trying to get one more game in the first set and make it more competitive, then one game turned into another and I was able to win that set," Gauff added.
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, ramping up the pace of her first serve and backhand. She was broken when serving for the match for the second time, before finally getting over the line.
Sabalenka storms into semi-finals despite late start
Gauff will play Sabalenka in the semi-final, a rematch of last year's final at Flushing Meadows in New York, after the defending champion breezed past Czech Barbora Krejcikova in straight sets.
The second seed has not lost more than three games in a set in any of her five matches so far and eased to a 6-2 6-3 win over the ninth seed.
The scheduling at the Australian Open again drew criticism as, after Gauff's earlier battle and a four-set win for Novak Djokovic in his quarter-final against Taylor Fritz, the evening session was over two hours late getting started.
Sabalenka and Krejcikova took to the court shortly after 9pm local time in Melbourne, having reportedly refused to move their match to the Margaret Court Arena when asked by tournament officials.
Sabalenka did her best to get the scheduled timings back on track, however, as she raced to victory in just one hour and 11 minutes.
Some trademark, thumping groundstrokes saw the 25-year-old quickly out into a double-break in both sets, and while Krejcikova managed to claw things back somewhat with a break of her own on each occasion, Sabalenka comfortably closed things out.
She broke again immediately in the first set before serving things out for a 6-2 lead, ultimately taking the second set 6-3 in similarly emphatic fashion.
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