Serena Williams has withdrawn from the Rome Masters tennis tournament, citing a persistent knee injury, which casts doubt on her participation in the French Open which begins this month.
The 37-year-old American world No.11 had been due to face sister Venus in an eagerly-awaited second-round clash in Rome.
Venus received a walkover into the third round.
Serena, a 23-times grand slam singles champion, has struggled throughout the year and completed only two matches since the Australian Open where she reached
"I must withdraw from the Italian Open due to pain in my left knee," Williams told the WTA website.
"I'll be concentrating on rehab and look forward to seeing you all at the French Open and next year in Rome."
Former No. 1 Caroline Wozniacki retired due to a left leg injury after losing a first-set tiebreaker to American Danielle Collins.
Denmark's Wozniacki, who also retired from her opening match at the Madrid Open last week, took a medical timeout at 5-2 down to receive treatment then came back to force the tiebreaker but quickly retired after losing the set.