Concerns for Peng's well-being arose after she went missing having made allegations that she had been sexually assaulted by Zhang Gaoli, a former senior vice-premier and high-ranking member of the Chinese Communist Party, on Chinese social media platform Weibo on November 2.

Following the incident of coercion, which happened 10 years ago, Peng said she had an on-off extramarital affair with Zhang.

The post was taken down after just 20 minutes, leading to accusations of censorship from the Chinese state.

The allegations were not reported on and became subject to blanket censorship in China.

Peng is reported to have gone missing after making the post.

Concerns for her safety increased when Chinese state media released a letter purporting to be Peng to Women's Tennis Association (WTA) chief executive and chairman Steve Simon.

Simon said the letter "only raises my concerns as to her safety and whereabouts" and that he has "a hard time believing that Peng Shuai actually wrote the email we received or believes what is being attributed to her."

In response, the IOC said "quiet diplomacy" is the best approach.

"Experience shows that quiet diplomacy offers the best opportunity to find a solution for questions of such nature," it told insidethegames.

"This explains why the IOC will not comment any further at this stage."

The upcoming Winter Olympics are due to be held in Beijing, the Chinese capital, despite criticism over China's record on human rights, press freedom and individual liberty.

Joining an increasing wave of social media calls for an investigation into where Peng is, posting on Twitter, Estanguet attached an image of Peng with the graphical tagline "#WhereisPengShuai".

The caption, translated from French, read: "We did the Beijing and London Games together.

"Today, like the whole sports community, I am worried about Peng Shuai. 

"We call for the greatest transparency on her situation."

Estanguet is a former IOC Athletes' Commission vice-chair and ex-IOC member.

The email it is claimed Peng wrote to Simon - a screenshot of which state-run media outlets have shared - begins "Hello everyone this is Peng Shuai".

A cursor, usually only present on the screen when someone is typing, is also visible, and the communication has been widely dismissed as not credible.

The email was only released on English-language platforms, while domestic Chinese media have not reported on its content.

In a statement in response to the letter's publication, Simon said: "The WTA and the rest of the world need independent and verifiable proof that she is safe.

"I have repeatedly tried to reach her via numerous forms of communication, to no avail.

"Peng Shuai must be allowed to speak freely, without coercion or intimidation from any source."

The WTA chairman is one of several key figures and organisations to have called for proof of Peng's whereabouts and safety, including the United Nations (UN), which demanded independent proof that Peng is safe and well.

"What we would say is that it would be important to have proof of her whereabouts and well-being, and we would urge that there be an investigation with full transparency into her allegations of sexual assault," Liz Throssell, the spokesperson of the UN Human Rights office, told reporters in Geneva, as reported by CNN.

"According to available information, the former world doubles number one hasn't been heard from publicly since she alleged on social media that she was sexually assaulted. 

"We would stress that it is important to know where she is and know her state, know about her well-being."

The Lawn Tennis Association - Britain's tennis governing body - and the National Olympic Committees of The Netherlands and Germany have also shared concern over Peng's safety.

"Clarity is needed about her well-being and current condition," said the German Olympic Sports Confederation.

American tennis star Serena Williams also posted on social media, using the same image as Estanguet.

Williams protested: "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. 

"Sending love to her and her family during this incredibly difficult time."

Naomi Osaka, Japan's four-time Grand Slam winner, Novak Djokovic from Serbia, the men's world number one, and American Chris Evert - a 20-time Grand Slam champion - have all voiced their worries.

Peng appeared at three Olympic Games - Beijing 2008, London 2012 and Rio 2016 - two of which Estanguet was also a competitor at.

China is due to host the Winter Olympics and Paralympics early next year.

Critics of the regime say China's record on human rights makes it an unfit Olympic host, and the Peng case is likely to give rise to more criticism over how China handles dissent.