Karolina Pliskova has saved four match points to send seven-time champion Serena Williams crashing out of the Australian Open.
Seventh-seeded Pliskova rallied from 5-1 down in a tense deciding set to advance to her first semi-final at Melbourne Park, with a 6-4 4-6 7-5 victory over the American on Wednesday.
"I was almost in the locker-room, but now I am standing here as a winner," Pliskova said after the 2h 10m battle on Rod Laver Arena. "It is a very good feeling.
"She was playing very well, especially at the end of the second set. She went for her shots and she was aggressive, and I played too passive.
"I said, 'Let's try this game 5-2, maybe I will have a couple of chances'. She got a little bit shaky in the end, so I took my chances and I won."
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The Czech will play Japanese fourth seed and reigning US Open champion Naomi Osaka in Thursday's semi-finals, while Williams remains stranded one Grand Slam shy of Margaret Court's all-time record 24 major singles titles.
Williams looked set would wrap up the match comfortably, up 5-1 in the third set, but she appeared to jar her ankle on her first match-point with Pliskova.
With the American seemingly troubled by the injury, Pliskova seized the opportunity to fight her way back into the match and proceeded to win the next six games to seal an unlikely victory.
Pliskova is unbeaten this year, after winning the season-opening Brisbane International.
The loss once again dashes Williams' hopes in Melbourne of matching Court's long-standing record, after the 37-year-old also fell just short at Wimbledon and the US Open as a beaten finalist.
Pliskova's previous best Grand Slam result was a loss in the 2016 US Open final to Angelique Kerber, while she's never previously gone further than the quarter-finals at Melbourne Park.
Earlier, Osaka closed in on a second successive Grand Slam title, sweeping past Elina Svitolina to book a semi-final berth.
The US Open champion blasted her way to a 6-4 6-1 win over the sixth-seeded Ukrainian, who sought treatment for a neck injury midway through the second set.
Osaka's win sets up a tantalising possible US Open final rematch with Serena Williams, who had a meltdown during that loss and who faces Karolina Ploskova in Wednesday's last women's quarter-final.
The biggest threat Osaka faced on Rod Laver Arena was when she was chased by a bee just before wrapping up the match, with Svitolina off her game.
Osaka, 21, said she tried to work on keeping her focus, after been taken to three sets in her last two matches.
"For me today, I just had one goal, and that was to try as hard as I can and not get angry," Osaka said.
"I didn't do that well in the last two rounds, so that was my only goal and I think I did it well, so I'm really happy with the way I played.
"She is a really great player and it is kind of unfortunate that she got injured, but playing against her, even when she was injured, was still really tough."
The world number four had too much power and poise for Svitolina, who was in good form heading into the match and high on confidence, after winning the WTA Finals title late last year.
But the Ukrainian struggled with her service game, only managing to get 58 percent of her first serves in.
The first set went on serve, before Osaka broke Svitolina to take a 4-2 lead, only to hand the break straight back through unforced errors.
The pair again traded breaks, before Osaka again got on top of her opponent and sealed the set on her fourth set-point.
Osaka won six straight games to take a 5-0 lead, before Svitolina managed to hold serve, but it only temporarily delayed the inevitable.
Svitolina's exit continued her modest record in Grand Slams, with the 24-year-old yet to reach a semi-final at a major.