Top seed Rybakina, who won her first six matches of 2024 without dropping a set, finally met her match on Thursday, thumped by Russian Ekaterina Alexandrova 6-3 6-3 in the quarter-finals.
The final scoreline flattered Rybakina, the 2022 Wimbledon champion and a finalist at Melbourne Park last year, who went 0-5 down in both sets at Memorial Drive.
The Kazakh's normally impenetrable serve was broken five times and her ball-striking was uncharacteristically loose.
Alexandrova will now take on Jelena Ostapenko for a spot in Saturday's final after the Latvian firebrand overcame Ukrainian prodigy Marta Kostyuk 7-5 6-3.
Latvian Jelena Ostapenko made it into the semis at Memorial Drive with a straight-sets victory. (Matt Turner/AAP PHOTOS)
The 26-year-old Ostapenko, who on the back of the win returns to the top 10 for the first time since 2018, fired 30 winners against the world No.41.
"It was a tough match, but finally I won in two sets, I managed it," Ostapenko said.
"I feel like I was striking the ball well today. Even in the first set when it went from 5-2 to 5-5, I still was playing my game, and then at the end I found it, so I'm just happy to be through."
Ostapenko showed no signs of the leg pains that hampered her previous match - a gruelling three-set win over France's Caroline Garcia - but the world No.12 did pull out of her doubles semi-final which was scheduled later that afternoon.
Tournament officials are confident Ostapenko will be fit to take on Alexandrova on Friday.
World No.5 Jessica Pegula recovered from a set and a break down to burst former French Open finalist Anastasia Pavlyuchenkova's bubble with a 6-7 (1-7) 7-5 6-4 victory.
The second-seeded American will face another Russian Daria Kasatkina, who was the beneficiary of a walkover against Germany's United Cup winner Laura Siegemund.
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