Serena Williams has withdrawn from next week's Madrid Open, saying she wasn't ready to compete.

Tournament organizers say Williams withdrew because she needs more time to train before returning to action.

Williams, a 23-time Grand Slam champion, returned to the tour in Indian Wells this year after a 14-month absence because of the birth of her daughter.

She won in Madrid in 2012 and 2013.

Williams's replacement in Madrid will be Coco Vandeweghe.