Five-time Grand Slam champion Maria Sharapova has withdrawn from Stanford's Bank of the West Classic before her scheduled second-round match.
Tournament officials said the 30-year-old had pulled out on her doctor's advice due to a left arm injury.
It means Ukraine's Lesia Tsurenko has progressed in California.
Sharapova, given a wildcard to play in the US for the first time since March 2015, had defeated Jennifer Brady 6-1 4-6 6-0 on Monday.
The Russian had previously been out with a thigh problem since 16 May.
Sharapova returned in April after a 15-month doping ban, but was denied a wildcard for the French Open earlier this year and was unable to take part in qualifying for Wimbledon because of injury.