Zhu, ranked No.48 in the world, has not lost a set through three Tennis in the Land rounds. She broke Garcia five times on Thursday to swiftly advance to the semi-finals in 85 minutes.
Earlier in the week, world No.7 Garcia snapped a four-match losing streak having lost in the first rounds of her last three tournaments in the American swing - first in Washington, then in Montreal and last week in Cincinnati.
"I'm very proud of my performance today because Caroline is an amazing player," Zhu said.
"She always hits a big serve, and I have to be ready and be more patient and stay in the game against her."
Fourth-seeded Russian Ekaterina Alexandrova will face Zhu in the semi-finals of the final US Open tune-up after rallying past Wang Xinyu 5-7 6-0 7-5.
The tournament was affected by multiple rain delays for the second straight day, forcing ninth-seeded Sloane Stephens to play two matches in an eight-hour span.
The American world No.38 defeated 16-year-old Mirra Andreeva of Russia 6-3 6-3 in the morning but lost 6-1 6-3 to Sara Sorribes Tormo in the quarters.
The Spaniard will next face Tatjana Maria after the German beat Leylah Fernandez of Canada 6-3 6-2 in the other evening match.
Both semis will be played Friday (Saturday AEST).