World number 26 Madison Keys was one of three seeded players to be eliminated in the second round of the Silicon Valley Classic in San Jose on Wednesday.
The American third seed was beaten by China's Zhang Shuai 7-5 7-6 (7-5) in one hour and 39 minutes.
Keys sent down 7-1 aces but Zhang, ranked 51st in the world, was excellent on return, converting four of her 11 break points.
Fourth seed Daria Kasatkina was also in danger after losing the first set but rallied to win 3-6 7-5 6-3 over France's Caroline Garcia.
"It was a tough match. From my point of view, I think the level of the game was very good, and I'm really happy that I was able to turn the match in the second set," Kasatkina said in the on-court interview.
Ninth seed Alison Riske was emphatically beaten by Croatian Ana Konjuh 6-1 6-4 in one hour and 13 minutes, while sixth seed Petra Martic bowed out to Poland's Magda Linette 7-5 7-6 (7-5).