Williams had Pliskova on the rack serving at 5-1 in the third set but the seventh seed fended off four match points to bounce back and end the American's latest bid for a record-equalling 24th Grand Slam.

"She just played lights out on match point, literally hitting lines," Williams said. "Just went for it, just went crazy on match point. She just played unbelievable."

The 37-year-old made a slow start and conceded the first set but had recovered and looked set for a hard-fought win before a desperate Pliskova went on the six-game rampage.

"She was killing me... I was almost in the locker room, now I'm standing here as the winner. It's a great feeling," an exhausted Pliskova said after the two hour 10 minute epic.

"My mind was in the locker room. I wasn't playing very well in the third set but I thought let's try, this game (will make it) 5-2. She got a little bit shaky in the end so I took my chances and I won."

Pliskova, who will play Japan's Naomi Osaka in the semis, was on the brink until Williams double faulted with the end in sight after appearing to roll her ankle.

Williams, a seven-time Australian champion, then failed to win a point on serve for the rest of the match.

She refused to blame the injury for the loss, saying her ankle "seems to be fine".

The elated Czech rated it the best comeback of her life and said she always believed she could beat the American legend.

"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," she said in explaining how she approached the match.

"I beat her before. She was No.1 at that time. I knew I'm going to have my chances. But I had to play well."

Williams said there was nothing she could do against an opponent in such form.

"I can't say that I choked on those match points, said the American, who made a comeback last year after having a baby. "She literally played her best tennis ever on those shots."

'Go bananas' 

Williams, seeded 16, had blazed through the early stages of the tournament, seeing off world number one Simona Halep in the fourth round.

But against Pliskova she lacked the intensity she had previously displayed and was soon trailing for the first time during her campaign after a slew of uncharacteristic errors.

The American said it was a lesson to not take anything for granted on match point. 

"This is when you go psycho, when you have match point... just go bananas on it," she said.

Pliskova, who entered this year's tournament on a high after winning the Brisbane International warm-up, came out firing and bested Williams on serve to take the first set.

Williams rebounded in the second before imploding in the decider.

Former world number one Pliskova has made the quarter-finals in her last two appearances at Melbourne Park and made the final at the US Open in 2016.

She has been a regular in the second week of Grand Slams in recent years, reaching the quarters or better six times since late 2016, but is still searching for a breakthrough win.

The Czech said the win over Williams was a huge confidence booster before facing US Open champion Osaka.

"She's dangerous but nobody is more dangerous than Serena," she said.

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