Sharapova, who was competing as a wildcard in her fourth tournament back after a 15-month doping ban, said she started to feel pain in her forearm late during her first-round win over American Jennifer Brady on Monday.
"After yesterday's scan, the doctor has recommended I don't risk further injury," Sharapova, who was awarded a wildcard to compete this week in California, said in a statement on the tournament's website.
"Monday night’s crowd was so special and I wish I could continue playing but I have to make a preventative decision.”
Sharapova, playing her first match in the United States since 2015, was made to work against world number 80 Brady, needing two hours to secure the 6-1 4-6 6-0 win.
Her early exit means Ukrainian seventh seed Lesia Tsurenko advances to the third round by walkover.
It marks the second time in three months that the five-times grand slam champion's comeback has been interrupted by injury as she also retired from her second round match at the Italian Open in mid-May after injuring her left thigh.
Sharapova, banned after testing positive for heart drug meldonium at the 2016 Australian Open, has had mixed results since re-joining the WTA Tour.
The 30-year-old Russian reached the semi-finals as a wildcard in Stuttgart but fell in the second round at Madrid to Canada's Eugenie Bouchard who days earlier had labelled the Russian a "cheater." — Reuters
