The 23-time Grand Slam champion suffered early exits at Indian Wells and Miami in her only two competitive outings this year following a 14-month layoff to give birth to daughter Alexis Olympia.

Williams has not been seen on the WTA Tour since losing to Naomi Osaka in the first round in Miami and has decided she needs more preparation time to compete on clay.

"We regret the absence of Serena," Madrid Open organizer Manolo Santana said on the tournament's official website. "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."

Williams won titles in Madrid in 2012 and 2013.

Williams also was scheduled to take part in a charity event in advance of the main tournament. Her place in the charity exhibition was taken by fellow American CoCo Vandeweghe.