Serena Williams pulled out of the Italian Open on Tuesday due to a persistent knee injury which casts doubt on her participation in the French Open which begins this month.
The 37-year-old American world number 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.
???I must withdraw from the Italian Open due to pain in my left knee,??? the 37-year-old said.
???I will miss the fans and competition at one of my favourite tournaments.
"I??ôll be concentrating on rehab and look forward to seeing you all at the French Open and next year in Rome."
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 the quarter-finals.