Madrid: World No.1 and former champion Serena Williams on Friday withdrew from this year's Mutua Madrid Open tennis tournament.

Williams was suffering from flu, ruling her out from returning to the tournament she won in 2012 and 2013, tournament organisers said.

"Unfortunately, I have to withdraw from Madrid because I've been battling the flu and I'm not at 100%. I look forward to returning soon," Williams said in a statement.

Williams, who is the defending French Open champion, is now scheduled to open her clay court season at the Internazionali BNL d'Italia in May.