Serena Williams has said she is "devastated and shocked" by the case of Chinese tennis star Peng Shuai, whose whereabouts have become a mystery sparking international concern.
Doubles grand slam winner Peng is said to have been missing since making sexual assault allegations against a former top Chinese government official.
She posted on Chinese social media site Weibo allegations against Zhang Gaoli – the ex-vice premier and member of the Chinese Communist Party's Politburo Standing Committee – claiming he had forced her to have sexual relations with him.
The head of the women's tour, WTA chairman and CEO Steve Simon, has questioned the veracity of an email purportedly written by the two-time major doubles champion saying that she is safe.
Now 23-time grand slam singles winner Williams has added her powerful voice, stating: "I am devastated and shocked to hear about the news of my peer, Peng Shuai. I hope she is safe and found as soon as possible. This must be investigated and we must not stay silent."
WTA boss Simon said he had "a hard time believing" the email in Peng's name had come from the 35-year-old player herself. The message stated the allegations of sexual assault were not true and that Peng was "resting at home".
Former world number one Williams said she was "sending love to her and her family during this incredibly difficult time", adding the hashtag "#whereispengshuai" to her Twitter message.
Williams and Peng have faced each other four times in their careers, with the American winning each of their singles matches, the most recent in 2014, while Peng and Sun Tiantian beat Williams and sister Venus Williams in doubles in Bangalore in 2008.
There has been concern throughout sport and beyond for the wellbeing of Peng, a French Open and Wimbledon doubles winner.
On Wednesday, Simon said it was important that there was "independent and verifiable proof that she is safe", saying the statement issued in her name "only raises my concerns as to her safety and whereabouts".
He said he had made efforts to contact Peng "via numerous forms of communication, to no avail", saying she must be allowed to speak "freely, without coercion or intimidation from any source".
Peng's social media post containing her allegations, along with all of her other content, has been removed from Weibo.
Four-time major winner Naomi Osaka also spoke up this week, with the Japanese star stating: "Censorship is never ok at any cost, I hope Peng Shuai and her family are safe and ok. I'm in shock of the current situation and I'm sending love and light her way."