Fourth-seeded Victoria Azarenka has pulled out of the Madrid Open because of a back injury.

The 26-year-old Azarenka says she "tweaked" her back during her opening match against Laura Robson and the pain persisted during her second-round win over Alize Cornet.

Azarenka had been scheduled to play Louisa Chirico of the United States today, but withdrew during the warmups.