Karolina Pliskova has saved four match points to send seven-times 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.
"I was almost in the locker-room but now I am standing here as a winner - it is a very good feeling," Pliskova said after the two-hour, 10-minute battle on Rod Laver Arena.
"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."
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 like she 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.