Serena Williams squandered four match points and a commanding lead in the deciding set as Czech seventh seed Karolina Pliskova produced a stunning comeback to claim a 6-4, 4-6, 7-5 quarterfinal win over the American tennis star at the Australian Open.

The 2016 US Open finalist was on the brink of defeat in front of a packed Rod Laver Arena in Melbourne on Wednesday, before securing a spot in her first semifinal at the tournament.

Williams, who is chasing a milestone 24th major triumph, was denied a chance to match Australian great Margaret Court's all-time record of Grand Slam singles titles.

While disappointed, the 37-year-old was philosophical in defeat and pledged to regroup for another tilt at the French Open.

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"It's definitely not easy for me. From day one, I expect to go out and, quite frankly, to win," the seven-time champion in Melbourne told reporters.

Serving for victory at 5-1, 40-30, in the third set, Williams, playing in her first Australian Open since becoming a mother in 2017, twisted her left ankle and dumped a forehand into the net.

Three more match points would follow while Pliskova served, and she saved each one.

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.

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.

"I think it's the best comeback ever so far in my life," an elated Pliskova told reporters.

"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 quarterfinal loss to Williams at last year's US Open and now holds a 2-1 Grand Slam record against the American, having beaten her in the 2016 semifinals at Flushing Meadows.

The 26-year-old former world number one, chasing her maiden Grand Slam title, is the second Czech into the semifinals after double Wimbledon champion Petra Kvitova thrashed home favourite Ashleigh Barty on Tuesday to book a match with Danielle Collins.