"I regret not having the opportunity to compete in the Western & Southern Open this year," Williams said. "I am making progress towards recovering from a viral illness that began affecting me last week, and have plans to return to the practice court in the coming days. I am optimistic that I will be ready for the US Open at the end of this month."
Kim Clijsters will also miss the tournament as an injury suffered in Toronto leaves her unable to defend her title.
"Unfortunately, I won't be able to make it to Cincinnati this year," Clijsters said. "I will not be ready because of the left abdominal injury that happened during my first round in Toronto. It is very disappointing because I love playing in Cincinnati and I was looking forward to defending my title. And Jada was also looking forward to it."
The withdrawal will affect the seeding for the 2011 tournament as each seeded player will move up one spot, helping Marion Bartoli the most as she will become the No. 8 seed and have a first round bye.
Additional player withdrawals from the women's field include Dominka Cibulkova, Kaia Kanepi, Alexandra Dulgheru, Bethanie Mattek-Sands and Tamira Paszek. These withdrawals have allowed Ekaterina Makarova, Lucie Hradecka, Anabel Medina Garrigues, Iveta Benesova, Lourdes Dominguez Lino, Ayumi Morita and Tsvetana Pironkova to join the WTA field.
On the men's side, Robin Soderling, Ivan Ljubicic, Milos Raonic, Tommy Robredo and Lleyton Hewitt have withdrawn. The following players have been added to the men's field: Juan Monaco, Sergiy Stakhovsky, Ivo Karlovic, Andrey Golubev and John Isner.
The main draws for the Western & Southern Open will be made at 6 p.m. Friday.