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."
Sharapova's withdrawal opened the way into the last eight for third-seeded Daria Kasaatkina, who enjoyed an opening round bye.
The former world number one Sharapova failed to reach the Australian Open quarter-finals earlier in January, but claimed she still believes she can increase her tally of five Slam titles.
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