From a set and a break down, the title favourite turned things around to lead 5-1 in the deciding set and had four match points but lost six games in a row to fall to a 6-4 4-6 7-5 defeat.
Williams had her first match point at 5-1 but, after being called for a foot fault on what appeared to be an ace, she jarred her left ankle and did not win another point on serve during the match.
The other three chances came with Pliskova serving at 4-5 but the Czech was rewarded for bravery on her forehand, and Williams insisted it had been her opponent who had won the match rather than she who had lost it. “There’s nothing I did wrong on those match points. I can’t say that I choked. She literally played her best tennis ever on those shots. I don’t really take losses well. But Karolina played lights out starting 5-1, 40-30. Literally, I’ve never seen anything like it. If anything, I think that’s a little bit easier to know, okay, next time I’m up 5-1 against anybody, I just need to make sure I play lights out when I have match points.
“I need matches to know that, okay, this is when you go psycho.”
Pliskova takes on fourth seed Naomi Osaka, who extended her winning run at the slams to 12 matches with a 6-4 6-1 victory over Elina Svitolina, who was hampered by pain in her shoulder and neck.
The Japanese player is in pole position to take over as world No 1 at the end of the tournament, although Pliskova and Petra Kvitova are also still in contention.