Serena Williams has withdrawn from next week's Madrid Open, citing the need to spend more time training before returning to competitive action.

A 23-time grand slam singles champion, Williams made her comeback earlier this year following the birth of her first child last September.

However, after reaching the last 32 in Indian Wells and making a first-round exit from the Miami Open in March, the 36-year-old has not featured in a WTA event.

She was set to compete in the Spanish capital, in an event she has won twice, only for her withdrawal to be confirmed on Thursday.

A statement on the tournament's official website read: "Serena Williams will not contest the Mutua Madrid Open 2018, [due to] needing a greater load of training to return to competition.

"The American has tried until the last minute to be ready to reappear on the clay of the Caja Magica."

"We regret the absence of Serena and we wish her a speedy recovery," said tournament director Manolo Santana.

"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."