Maria Sharapova fell to a shock defeat at the Rogers Cup in Montreal.
The French Open champion suffered a setback in her build-up to the US Open, losing in three sets to Spain's Carla Suarez Navarro.
Sharapova went down 2-6 6-4 2-6 to the number 14 seed in a rain-hit last-16 match.
The 27-year-old said: "I couldn't find my rhythm from the beginning and always had my back against the wall."
The Russian refused to use the conditions as an excuse for her unexpected exit at the hands of her 25-year-old opponent.
The final Grand Slam of the year starts at Flushing Meadows on August 25, and the former Wimbledon champion added: "There's quite a bit of time until then. It's just a matter of fine-tuning a few things."