Alexandrova defeated the four-time Grand Slam winner from Belarus 7-6 (7/3), 4-6, 6-4 in 2hrs 29mins to book a berth in the quarter-finals.
Sabalenka, who will seek her third consecutive US Open title next month, fell to 5-5 in her career WTA rivalry with Alexandrova, who will face either American eighth seed Amanda Anisimova or Ukraine’s ninth-seeded Elina Svitolina in the quarter-finals.
Sabalenka broke for a 2-0 lead in the opening set, dropped the next four games, then held and broke back to 4-4 on a forehand drop volley winner.
Double faults by Sabalenka in the 11th game and Alexandrova in the 12th surrendered breaks leading to a tie-break.
The Russian took five of the last six points to seal the first set when Sabalenka swatted a crosscourt forehand wide.
In the second set, Alexandrova hit a forehand long to surrender a break and give Sabalenka a 4-3 lead.
The top seed then held serve twice to force a decider.
Sabalenka was broken when she netted a forehand to give Alexandrova a 3-1 edge, but the Russian double-faulted away a break in the seventh game.
In the 10th game, Sabalenka made three unforced errors to give Alexandrova break and match points.
Sabalenka saved the first on a service winner and the second on an ace.
But after sending a backhand beyond the baseline, the Belarusian made her sixth and final double fault to lose the match.
“I didn’t have any expectations before the match,” Alexandrova said.
“I knew it was going to be super difficult when I walked on the court.
“I was trying to risk as much as possible because it felt like it was the only way to create some chances. It caused many more mistakes but at the same time I created many more opportunities.” (AFP)
