Serena Williams has withdrawn from the Italian Open with left knee pain but is confident of being fit for the French Open.
The 23-time grand slam singles champion had been due to face sister Venus for the 31st time after beating Rebecca Peterson in the first round on Monday.
Williams said on wtatennis.com: "I must withdraw from the Italian Open due to pain in my left knee. I will miss the fans and competition at one of my favourite tournaments.
— WTA (@WTA) May 14, 2019
"I'll be concentrating on rehab and look forward to seeing you all at the French Open and next year in Rome."
The Italian Open was only Williams' fourth tournament of the season and she will head to Paris having played just eight completed matches in 2019.
The 37-year-old took nearly two months off after withdrawing ahead of her second match in Miami in March, also with knee pain, while she retired mid-match in Indian Wells earlier that month because of illness.
The only other tournament Williams has played was the Australian Open in January, where she led Karolina Pliskova 5-1 in the deciding set in the quarter-finals only to suffer the most dramatic collapse of her career.
Williams, currently ranked 11th, made her grand slam return in Paris 12 months ago following the birth of her daughter and reached the fourth round before withdrawing ahead of a much-anticipated clash with Maria Sharapova because of a shoulder problem.
The French Open begins on May 26.