Former world number one Serena Williams has withdrawn from next week's Italian Open, the organisers said on Wednesday, casting doubts over her participation in the French Open beginning later this month.

The 36-year-old, who has triumphed in Italy on four previous occasions, returned to action in March after the birth of her daughter last September but is yet to regain full fitness.

The 23-times Grand Slam winner made her comeback at Indian Wells in March and has not played since her opening round defeat by Japan's Naomi Osaka at the Miami Open.

"We are so sad to announce that Serena Williams, a four-time champion in Rome, has withdrawn. Obviously, we can't wait to see her again on Foro Italico's red clay, perhaps in 2019?," the organisers said on Twitter.

The Italian Open is held from May 14 to 20, a week before the French Open main draw kicks off on May 27.