"Oh my God! I have no words at this moment," said a beaming Ruse. "It's just a dream for me."
Ruse, ranked 122nd in the world, pulled off the biggest upset of the tournament so far with her win over the eighth-ranked Krejcikova.
The Czech, eliminated in the quarter-finals of the Paris Olympics, opted not to play any hard-court tune-ups heading into the US Open, withdrawing from Toronto with a thigh injury.
She acknowledged before the tournament that obligations in the wake of her Wimbledon victory "took a lot of energy," although adding a second major after her 2021 French Open victory had boosted her confidence and sense of calm.
The 26-year-old Ruse, who thanked her team for seeing her through a series of injuries, next faces 26th-seeded Paula Badosa of Spain, a 6-3, 7-5 winner over American Taylor Townsend.
"I don't care, to be honest right now. I just want to enjoy," said Ruse, who was especially pleased that her father, Adrian, was on hand to watch her play in the United States for the first time.
"I think my father is the lucky charm, for sure," she said.
