Maria Sharapova's latest return to the WTA Tour at the Bank of the West Classic came to a premature end on Wednesday due to an arm injury.
The Russian missed eight weeks of action with a leg injury before returning with a three-set victory over Jennifer Brady on Monday.
However, Sharapova was forced to pull out of the event in Stanford, where she was due to meet Lesia Tsurenko in round two, due to pain in her left forearm.
A statement published on the WTA's website read: "Unfortunately, I have to withdraw from today's match.
"Toward the end of Monday night's match, I felt pain in my left forearm. After yesterday's scan, the doctor has recommended I don't risk further injury.
"Monday night's crowd was so special and I wish I could continue playing, but I have to make a preventative decision."
Sharapova had made her return from a 15-month doping ban in April before injury ruled her out of the grass-court season.
She is scheduled to play in the Rogers Cup in Toronto and Western and Southern Open in Cincinnati this month, but will have to go through qualifying to play at the US Open if a wildcard is not offered.