(Corrects sport name in headline)
MADRID May 5 (Reuters) - Former world number one Venus Williams has been forced to withdraw from the Madrid Open due to a back problem, organisers said on Sunday.
The 32-year-old American, now ranked 21, was due to play her first-round match against Spanish wild card Anabel Medina Garrigues on the clay at the premier event on Sunday.
She will be replaced in the draw by 56th-ranked Stefanie Voegele of Switzerland. (Reporting by Iain Rogers; Editing by John O'Brien)