A day after women's champion Coco Gauff crashed out and hot on the heels of the exit of men's giants Jannik Sinner and Novak Djokovic, third seed Swiatek was simply swept aside in her fourth-round clash 7-5 6-1 by the Ukrainian on Court Philippe Chatrier.
After two title wins already this clay-court season in Rouen and Madrid, Kostyuk reeled off a 16th successive victory on the circuit, looking a genuine contender to lift her first grad slam crown as she stormed into the quarter-finals.
Swiatek's birthday celebrations quickly turned sour after she succumbed in a tight first set when she and Kostyuk twice traded breaks.
The 15th-seeded Kostyuk moved into overdrive in the second set once she'd saved three break points to move 2-1 ahead.
From that point, she lost just three more points, hitting a purple patch that left the Pole powerless with a host of searing winners - 25 in all over the one-hour 39-minute contest.
The victory left 23-year-old Kostyuk stunned as she'd never taken a set off Swiatek in their three previous meetings.
"I'm in shock," she told the crowd.
"To beat such an unbelievable player who won four times here, I'd never taken a set off her; I still cannot believe it,.
"I think the most important thing I've been doing is really just trying to enjoy. I woke up this morning and I just thought about what an unbelievable day I have to live today to play on Chatrier against Iga."
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