– Serena Williams was one point — just one — from quite a comeback victory in the Australian Open quarterfinals when she turned her left ankle.

Everything unraveled from there.

In a startling reversal and result, Williams wasted four match points along the way to dropping the last six games of a 6-4, 4-6, 7-5 loss to No. 7 seed Karolina Pliskova of the Czech Republic at Melbourne Park on Wednesday.

"I was almost in the locker room," said Pliskova, who trailed 5-1 in the third set, "but now I'm standing here as the winner."

So instead of Williams moving closer to an eighth championship at the Australian Open and record-tying 24th Grand Slam title overall, it is Pliskova who will continue her pursuit of her first major trophy.

In the semifinals, Pliskova will face No. 4-seeded Naomi Osaka, who advanced by beating No. 6 Elina Svitolina 6-4, 6-1 earlier Wednesday.

Williams' surprising loss scuttled a much-anticipated rematch after her loss to Osaka in the chaotic U.S. Open final last September.

The 37-year-old American's match against Pliskova was played under a stifling sun, with the temperature around 80 degrees, and Williams often stepped into the patches of shade behind each baseline. She did not start well, not well at all, and her mistakes were mounting and deficit was growing.

Only after trailing by a set and a break at 3-2 in the second did Williams seem to get going. From there, she immediately earned her first break point of the match and converted it to get to 3-all, beginning a run in which she claimed nine of 11 games.

That seemed to put her in control. And then it all changed. Serving for the victory at 5-1, 40-30, Williams was called for a foot fault — reminiscent of an infamous such call at the U.S. Open a decade ago. During the ensuing point Wednesday, Williams twisted her left ankle and dumped a forehand into the net.

She grabbed at her foot right afterward, and would go on to cede that game.

Not a big deal, right? She still had a sizable lead.

Except three more match points would follow, and Pliskova staved off each one.

"She got a little bit shaky in the end," Pliskova said. "So I took my chances. And I won."

Tiafoe's run halted

Rafael Nadal is back to feeling healthy. Probably not a coincidence that he's back in the Australian Open semifinals.

Playing his familiar brand of court-covering, ball-bashing, opponent-frustrating tennis, Nadal claimed 20 of his first 23 service points and saved the only two break chances he faced, ending American Frances Tiafoe's best Grand Slam run with a dominating 6-3, 6-4, 6-2 victory Tuesday night.

Nadal, 32, reached his 30th major semifinal and prevented Tiafoe from getting to his first, two days after he turned 21.

"I knew he was going to bring crazy intensity. I knew the ball was going to be jumping. I knew if he got hold of a forehand, it was going to be barbecued chicken," Tiafoe said. "But point in, point out, I've never seen someone so locked in."

Now Nadal goes up against another opponent much younger than he is, 20-year-old Stefanos Tsitsipas, who upset Roger Federer in the fourth round.

The 14th-seeded Tsitsipas became the first player from Greece to earn a semifinal berth at a major, beating No. 22 Roberto Bautista Agut 7-5, 4-6, 6-4, 7-6 (2) earlier Tuesday.

"It's going to be interesting," Tsitsipas said about his matchup against Nadal. "I feel all right with my game."

Collins reaches semifinal

In women's action, unseeded 25-year-old Danielle Collins of the U.S. reached her first Slam semifinal with a 2-6, 7-5, 6-1 victory against Anastasia Pavlyuchenkova of Russia. Collins was an NCAA champion at the University of Virginia who began this tournament with an 0-5 record at majors and now has strung together five victories in a row, including over 2016 champion Angelique Kerber.

Collins will face two-time Wimbledon champion Petra Kvitova, who is back in the semis at a major for the first time since she was stabbed during an attack at her home in December 2016.

"I didn't really imagine being back," a teary Kvitova said after defeating No. 15 Ash Barty of Australia 6-1, 6-4.

"I'm calling it my 'second career,' " Kvitova said. "So it's the first semifinal of the 'second career.' "