Serena and Venus Williams withdrew from the hard-court tournament in Cincinnati on Tuesday, becoming the latest high-profile tennis players to pull out of the US Open tune-up.
The Williams Sisters, and American Sofia Kenin, joined Roger Federer and Novak Djokovic as stars who will be giving the ATP and WTA tournament in Ohio a miss.
"Unfortunately, I will not be playing in the Western & Southern Open next week as I am still in recovery from my leg injury at Wimbledon," she said in a statement.
"I will miss all of my fans in Cincinnati whom I look forward to seeing every summer. I plan to be back on the court very soon."
The reason for Venus' withdrawal was not disclosed, but Kenin said she was pulling out with a foot injury "in hopes to play the US Open later this month".
"I feel another week of recovery is necessary," she added.
Swiss great Federer, a seven-time Cincinnati champion, said last week he would skip the tournament because of the same knee injury that kept him out of the Tokyo Olympics.