Second alternate Alexandrova had sat on the sidelines all week but her patience proved worthwhile on Wednesday when Keys – unable to advance – withdrew just hours before her match with Rybakina.
The 30-year-old, who has enjoyed a breakthrough year in 2025, received the nod after fellow Russian and first option Mirra Andreeva, who is also competing in the doubles, declared she was not fit to play.
Alexandrova started impressively but squandered three break points before returning a forehand wide to hand the first break and a 5-4 lead to Rybakina.
The big-hitting Rybakina, sporting tape on her serving shoulder, served out the first set to love before breaking early in the second courtesy of a backhand error off her opponent’s racquet.
As Alexandrova’s serve faltered, Rybakina stepped up a gear and she doubled her advantage with a brutal forehand winner on break point, only to immediately lose one of her breaks when serving for the match.
Her struggle to get over the finish line continued, forced to save two break points in her next service game, before eventually sealing victory on her second match point as Alexandrova sent a backhand long.
“Each win gives you confidence,” said Rybakina, 26. “Hopefully I can continue.”
In the doubles, 2022 champions Veronika Kudermetova and Elise Mertens confirmed their semi-final berth with a 6-3 6-3 victory over Italian pair Paolini and Sara Errani.
They join Hsieh Su-Wei and Jelena Ostapenko in advancing from the Martina Navratilova Group.