Madrid [Spain], May 4:  American tennis ace Serena Williams has pulled out of next week's Madrid Open, saying that she needs more time to prepare for the clay-court season.

Confirming the news, tournament director Manolo Santana said that Serena is one of the greatest tennis champions and that they regret her absence in the tournament.

"We regret the absence of Serena and we wish her a speedy recovery. She is one of the great champions of our sport, she has always offered a show to the tournament and we hope to see her again at the Caja Magica in the future," Santana was quoted as saying by WTA website.

Serena ended her 15-month absence from tennis during which she gave birth to daughter Alexis Olympia Ohanian Jr in March this year. She went on to play at Indian Wells and Miami Open but has admitted to struggling to get fully fit.

The 23-time Grand Slam champion is a two-time champion in the Spanish capital. She clinched the Madrid Open title in the 2012 and 2013 editions of the tournament.