Maria Sharapova's ongoing right shoulder problems led her to pull out of the Saint Petersburg tournament, the organisers said on Wednesday.
The 31-year-old has been struggling back to elite fitness since watching from the sidelines during a 15-month doping ban ended in 2017.
The five-time major winner beat Daria Gavrilova of Australia 6-0, 6-4 in her debut in the tournament, but suffered with her shoulder injury afterwards.
"I have worked hard for months to put it (her shoulder) in order, but to no avail," she said after her match with Gavrilova.
"I still have plenty of work to do."