Serena Williams withdraws from Madrid Open

— -- Serena Williams has pulled out of next week's Madrid Open because of illness.

The world No.1 said in a statement on the tournament's website: "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."

Williams has played only three tournaments this season and is yet to win a title, losing in the final of the Australian Open and at Indian Wells before a fourth-round exit in Miami a month ago.

Should the 34-year-old also miss the tournament in Rome in a fortnight, she would go into the French Open without having played on clay since last summer.

After coming so close to winning the calendar Grand Slam last season, Williams' dominance appears to be on the wane, although she still has a healthy lead in the rankings.

Agnieszka Radwanska will take over as top seed for the tournament ahead of Australian Open champion Angelique Kerber.

The news comes the day after Roger Federer confirmed he would play in Madrid having previously kept his options open.

Play begins at the Caja Magica on Saturday, with Andy Murray and Petra Kvitova defending their titles.

Murray warmed up for the tournament by practising in Majorca with Rafael Nadal, whom he beat in the final 12 months ago to win his first Masters title on clay.