Serena Williams squandered four match points and blew a 5-1 lead in the deciding set as seventh seed Karolina Pliskova produced a fightback for the ages to claim a 6-4, 4-6, 7-5 quarter-final win over the American great at the Australian Open on Wednesday.
Williams, chasing a milestone 24th Grand Slam triumph, also saved two match points in the last game but netted on the third as the big-serving Czech shocked a packed Rod Laver Arena with her remarkable rally from the brink of defeat.
The American's hopes of matching Margaret Court's all-time record of Grand Slam singles titles, and claiming a first since becoming a mother in 2017, were denied once more, after she also lost in finals at Wimbledon and the US Open last year.
While disappointed, the 37-year-old was philosophical in defeat and pledged to regroup for another tilt at the French Open.
"It's definitely not easy for me. From day one, I expect to go out and, quite frankly, to win," the seven-times champion in Melbourne told reporters.
"That hasn't happened. But I do like my attitude... I've only been playing 10 months, but I expect to win, and if I don't, it's disappointing.
"I rather think of it that way and know that it's going to happen sooner or later than making an excuse for myself. I don't like making excuses."
The brave Pliskova advanced to meet Naomi Osaka in the last four, denying Williams a chance to avenge her controversial US Open final defeat to the Japanese fourth seed.
"Normally I'm having a lot of comebacks, but maybe from a set down or a break down, so nothing like this. I think it's the best comeback ever so far in my life," an elated Pliskova told reporters after qualifying for her first Melbourne semi-final.
"This time I really believed, like, (she's) still a player, I know she's the greatest ever, but I don't want to put her somewhere where I would not have a chance to beat her."
Pliskova avenged her quarter-final loss to Williams at last year's US Open and now holds an enviable 2-1 Grand Slam record against the American, having beaten her in the 2016 semi-finals at Flushing Meadows.
The 26-year-old former world number one, chasing her maiden Grand Slam title, is the second Czech into the semi-finals after double Wimbledon champion Petra Kvitova thrashed home favourite Ashleigh Barty on Tuesday to book a match with Danielle Collins.
Locked in a three-way battle with Osaka and Kvitova to take the world number one spot from Simona Halep, Pliskova will emerge as the top-ranked woman if she claims the title.
Williams, seeded 16th as she continues her return from maternity leave, rolled her ankle when serving for the match at 5-1.
She promptly double-faulted after missing a passing shot on her first match point and was broken after dumping a backhand into the net.
Pliskova held on grimly, saving three more match points when serving to stay alive at 5-4, before breaking Williams to love and charging to 40-0 in the final game.
Williams saved two of the match points, the second with a courageous backhand winner, but Pliskova held firm to add another highlight to a tournament laden with upsets.
The American said her ankle was not a factor in defeat, and credited Pliskova for playing "unbelievable" on match points.
"She literally played her best tennis ever on those shots," she said.
Osaka in Aus Open semis for first time
US Open champion Naomi Osaka set up a potential rematch of her controversial Flushing Meadows final against Serena Williams after beating Elina Svitolina 6-4, 6-1 to reach her first Australian Open semi-final on Wednesday.
The 21-year-old Osaka committed a string of unforced errors but still proved too strong for the sixth-seeded Ukrainian, wrapping up the win in an hour and 12 minutes to become the first Japanese woman to reach the last four at Melbourne Park since Kimiko Date in 1994.
Osaka's first Grand Slam triumph at the US Open last year was overshadowed by a row between Williams and umpire Carlos Ramos. The American is playing Karolina Pliskova in the remaining quarter-final.
"I played them both and they are both very great players," said Osaka after her match at Rod Laver Arena.
"And I know that it's going to be tough no matter who I play, and honestly I'm just trying to go inside because it's little bit hot right now. Sorry," she added as the crowd burst into laughter.
Osaka's power was on full display against WTA Finals champion Svitolina, with the Japanese thumping 31 winners to her opponent's 11. But she was also sloppy at times, committing 25 unforced errors, nine more than Svitolina.
The 24-year-old Ukrainian kept her errors in check to get two service breaks back and stay in the opening set at 4-5.
Osaka blew three set point opportunities on Svitolina's next serve with three embarrassing errors, berating herself with angry slaps on the thigh.
However, the Japanese gathered herself and hit a crisp backhand to set up a fourth chance, and converted it when Svitolina thumped a return into the net.
The Ukrainian took a medical timeout in the second set for neck and shoulder problems with Osaka up 3-0 but she was broken again after returning to the court.
"I felt pain from the beginning of the tournament here and there. It was going and coming back," Svitolina said.
"Unfortunately today was a little bit worse than I expected.
"It's normal to have some tension during the Grand Slam and it happens, but unfortunately I couldn't handle it today."
Fourth seed Osaka served her eighth ace then converted her first match point with an overhead winner to seal the win.