(AFP) - World No. 1 Serena Williams has withdrawn from the Madrid Open because of flu.
The 21-time Grand Slam champion has limited her schedule to just three tournaments so far this season despite missing out in the final of both the Australian Open and Indian Wells before crashing out at the fourth round of the Miami Open last month.
The Madrid Open will feature several top stars from the women's game.Williams's conqueror at the Australian Open, Angelique Kerber, is in fine form after backing up her first Grand Slam by retaining her title on the indoor clay of Stuttgart last week.
Two-time champion Petra Kvitova returns to defend her crown at the Caja Magica and strongly believes the conditions favour her style.
World No. 5 Victoria Azarenka is the most in-form player on the Tour as the Belarusian arrives in Madrid on the back of a 14-match winning streak having won in Indian Wells and Miami, as well as helping her country relegate Russia in the World Group II in the Fed Cup earlier this month.
Home hopes are carried by Garbine Muguruza as the world No. 4 hopes to become the first Spanish woman to win the Madrid Open.
However, Muguruza has had a disappointing season so far as she has failed to make it beyond the quarter-finals in seven tournaments.