Serena pulls out of Madrid event

World number two Serena Williams has withdrawn from the Madrid Open due to a knee injury.

Williams, seeded second at the event behind Dinara Safina, made the decision to retire after losing the first set of her opening round match against Francesca Schiavone 6-4 this afternoon.

The 27-year-old Williams won her 10th grand slam singles title at the Australian Open earlier this year.