Madrid - World No 1 Serena Williams has pulled out of the Madrid Open, which begins this weekend, with flu, organisers announced on Friday.
"Unfortunately, I have to withdraw from Madrid because I've been battling the flu and I'm not at 100 percent. I look forward to returning soon," said Williams in a statement released by organisers.
The 21-time Grand Slam champion, 34, is a two-time winner in Madrid having triumphed in the Spanish capital in 2012 and 2013.
Last year she reached the semi-finals before losing to eventual winner Petra Kvitova, of the Czech Republic.
In the absence of Williams, Poland's Agnieszka Radwanska, who had been seeded two, will become the top seed.
Williams, who is the defending French Open champion, is now scheduled to open her clay-court season at the Italian Open in May.