It's just the latest disappointment for Iga Swiatek, who was eliminated in the quarter-finals at the Australian Open and in the WTA 1000 events in Doha and Indian Wells.
Ninth-seeded Ukrainian Elina Svitolina knocked off second-seeded Iga Swiatek of Poland 6-2, 4-6, 6-4.
Two-time champion Iga Swiatek barreled into the Indian Wells quarter-finals on Wednesday, beating in-form Czech Karolina Muchova 6-2, 6-0.
World number one Aryna Sabalenka on Tuesday voiced her support for a proposal to make women's matches best-of-five sets during the later stages of Grand Slam tournaments.
Swiatek, who won the WTA 1000 tournament in 2022, 2023 and 2024, had breezed through her last six meetings with Kasatkina without dropping a set but blew a 5-3 lead to lose the opener.
Australian Open chief Craig Tiley said it was "a fine line" between player promotion and privacy after top stars complained about backstage cameras at the Grand Slam.
Coco Gauff called herself "a real person with real feelings" as fellow players backed a call for more privacy during tournaments, with Iga Swiatek saying they felt like "animals in the zoo".
Fifth seed Elena Rybakina stunned world number two Iga Swiatek to race into the Australian Open semi-finals Wednesday.
Six-time major champion Iga Swiatek ended the fairytale run of qualifier Maddison Inglis with a rampant display at the Australian Open on Monday to set up a quarter-final with fifth seed Elena Rybakina.
World No. 2 Iga Swiatek registered a 6-2, 6-3 straight-set victory at the Australian Open on Thursday, reaching the third round in her 24th consecutive Grand Slam.