Camila Giorgi missed out on a meeting with Maria Sharapova as she was beaten by Alison Riske in round two.
The Italian suffered a crucial break in the first set as she went down 7-5 in 52 minutes.
Riske stepped up a gear in the second set which she won 6-2 in 32 minutes.
Giorgi served three aces as well as seven double faults, while a total of 21 unforced errors halted her progress.
There was also disappointment for Scott Lipsky in the Men's Doubles. He and Santiago Gonzalez went down in a five-set thriller against Leander Paes and Radek Stepanek (CZE) in round two.
Lipsky and Gonzalez led the fifth seeds by two sets to one, before bowing out 11-9 in the decider.