Saint Petersburg - Maria Sharapova's ongoing right shoulder problems led her to pull out of the Saint Petersburg tournament, the organisers said on Wednesday.
"I tried to do everything possible to be able to be ready to compete this week, but my right shoulder continues to be an issue for me," Sharapova said to explain her decision.
"I will meet with my team of doctors over the next few days to evaluate the situation, and I look forward to returning to completion as soon as possible."
The 31-year-old has been struggling back to elite fitness since watching from the sidelines during her 15-month doping ban that 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 Russia's Daria Kasatkina, 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.