Women's Tennis Association

In a post on Chinese social media site Weibo, Peng said she was "forced" into a sexual relationship with Zhang Gaoli.

"The events concerning Peng Shuai are of deep concern," said the WTA.

"The allegations must be investigated fully, fairly, transparently and without censorship."

Peng, 35, is a former world number one doubles player who has won two Grand Slam titles - at Wimbledon in 2013 and the 2014 French Open, both alongside Taiwan's Hsieh Su-wei.

Muguruza becomes only the second Spanish woman after Arantxa Sanchez Vicario to become the top-ranked player in the world.

The 24-year-old became the first player to beat both Serena and Venus Williams in major finals, having beaten Serena to claim the 2016 French Open.

"Becoming the WTA number one in the world is a dream come true," Muguruza told the WTA's website.

"Every birthday wish was always the same as I blew out the candles - to become the best.