World No 1 Elena Rybakina could not regroup and fell to 118th-ranked Alina Charaeva in the second round of the China Open on Saturday in Beijing.
The 27-year-old from Kazakhstan was playing her first match since seizing the top ranking on her US Open win in New York, but Armenia's Charaeva triumphed 3-6, 6-4, 6-3 in a clinical comeback.
"I don't have words," said Charaeva, who is making her Beijing debut.
"This is incredible," the 24-year-old said.
"She's (an) amazing player, and for me it's kind of just dream come true," she said of Rybakina.
"I will be really happy to stay some more days in Beijing," she added. "Never stop believing, I guess."
Rybakina got off to a solid start, but seven double-faults ushered the momentum into the hands of her opponent.
Wind rattled the closed-roof stadium, with both players serving as thunderous claps echoed over the court.
A supportive crowd tried to rally Rybakina back into contention, but Charaeva held serve to win the match in two hours and 11 minutes.
Charaeva faces Briton Sonay Kartal in the third round.