WTA chairman and CEO Steve Simon says he is even more concerned about Peng Shuai after receiving an email purportedly written by the two-time major doubles champion saying she is safe.

Peng has reportedly 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.

Chinese state media sent a statement to Simon with an email supposedly written by Peng stating the allegations of sexual assault were not true and that she is "resting at home."

The WTA boss on Wednesday reacted by saying he has "a hard time believing" Peng wrote the email and he is even more worried about the 35-year-old.

"The statement released today by Chinese state media concerning Peng Shuai only raises my concerns as to her safety and whereabouts," he said in a statement.

"I have a hard time believing that Peng Shuai actually wrote the email we received or believes what is being attributed to her. Peng Shuai displayed incredible courage in describing an allegation of sexual assault against a former top official in the Chinese government.

"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. Her allegation of sexual assault must be respected, investigated with full transparency and without censorship.

"The voices of women need to be heard and respected, not censored nor dictated to."

The WTA called for a full investigation into the matter last week.

Peng's social media post, along with all of her other content, has been removed from Weibo.

A spokesperson for the ministry of foreign affairs informed reporters he was not aware of the situation.

"I have not heard of the issue you raised," the spokesperson said via a widely released statement. "This is not a diplomatic question."

Naomi Osaka has expressed her concern over the situation of Peng Shuai, who has not been heard from publicly since accusing China's ex-vice premier of sexual assault.

Peng, a Wimbledon doubles champion in 2013, posted allegations against Zhang Gaoli on Chinese social media site Weibo last week.

The post, along with all of Peng's other content, has since been removed from the site.

Multiple reports suggested the 35-year-old had subsequently not been seen or heard from, and the Women's Tennis Association (WTA) called for a full investigation into the matter.

There has so far been no response from the Chinese government to the allegations.

However, a spokesperson for the ministry of foreign affairs informed reporters he was not aware of the situation.

"I have not heard of the issue you raised," the spokesperson said via a widely released statement. "This is not a diplomatic question."

Now Osaka, a four-time major champion and one of the biggest names in the sport, has used her platform to question the situation.

"Hey everyone, not sure if you’ve been following the news but I was recently informed of a fellow tennis player that has gone missing shortly after revealing that she has been sexually abused," Osaka wrote in a statement published to her official Twitter account.

"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."

Osaka concluded her post with the hashtag: "#whereispengshuai".

On Sunday, WTA chairman Steve Simon addressed the matter in a statement that said: "The recent events in China concerning a WTA player, Peng Shuai, are of deep concern.

"As an organisation dedicated to women, we remain committed to the principles we were founded on – equality, opportunity and respect.

"Peng Shuai, and all women, deserve to be heard, not censored. Her accusation about the conduct of a former Chinese leader involving a sexual assault must be treated with the utmost seriousness.

"In all societies, the behaviour she alleges that took place needs to be investigated, not condoned or ignored. We commend Peng Shuai for her remarkable courage and strength in coming forward.

"Women around the world are finding their voices so injustices can be corrected.

"We expect this issue to be handled properly, meaning the allegations must be investigated fully, fairly, transparently and without censorship.

"Our absolute and unwavering priority is the health and safety of our players. We are speaking out so justice can be done."

Osaka's comments follow on from men's world number one Novak Djokovic expressing his concern over the situation when speaking to the media earlier this week.

Djokovic said: "I don't have much information about it. I did hear about it a week ago.

"Honestly, it's shocking that she's missing, more so that it's someone that I have seen on the tour in the previous years quite a few times.

"It's not much more to say than hope that she will be found, that she's OK. It's just terrible. I can imagine just how her family feels that she's missing."

Novak Djokovic "feels amazing" after surpassing Pete Sampras for the year-end number one record on the ATP Tour.

Djokovic was presented with the trophy for finishing the year ranked number one for the seventh time after defeating Casper Ruud 7-6 (7-4) 6-2 in his ATP Finals opener on Monday, breaking a tie with idol Sampras.

A 20-time grand slam champion – a joint record held alongside Rafael Nadal and Roger Federer, Djokovic revelled in his latest achievement.

"It feels amazing and it feels even better when you win a match and then get your hands on the trophy that I have been blessed to lift seven times," Djokovic said in Turin.

"[I have won] one more than Pete Sampras, who was my childhood hero. He was the one that got me into tennis. He was also an inspiration to me and I dreamt of being a Wimbledon champion and World No. 1 like he was.

"Fast forwarding to today, it is quite amazing to be in this position. I am very grateful. It is something I am very much appreciating and not taking for granted."

World number one Djokovic is looking for his sixth ATP Finals title, though his last success came in 2016.

Djokovic produced a confident performance in his opening match against Ruud, dropping just four points behind his first serve and rallying from a break down in a tight opening set.

The top seed – who tied Ivan Lendl for second-most wins (39) in the history of the ATP Finals – will also face Stefanos Tsitsipas and Andrey Rublev in the Green Group.

"It was a really terrible start, but also funny because I’m still trying to figure out what happened," Djokovic said. "Casper started strong. He was serving well. The altitude, fast court, fast balls – it favours big servers. I knew he had a solid serve, but maybe not as good as Medvedev or Zverev. 

"He did positively surprise me with this serve, particularly in the first set. I just managed to read it better in the second set. But it was a close one."

Djokovic, meanwhile, expressed his shock amid the unknown whereabouts of WTA player Peng Shuai.

The Women's Tennis Association (WTA) has opened an investigation into Peng's sexual assault allegations against a former China leader.

Shuai, 35, 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.

In the post, published on Tuesday before subsequently being deleted, the 2013 Wimbledon doubles champion alleged that the pair had extramarital relations and she had developed feelings for him.

All of Peng's content has since been removed from Weibo and numerous reports suggest she has not been seen in recent days.

"I did hear about it a week ago. Honestly, it's shocking that she's missing, more so that it's someone that I have seen on the tour in the previous years quite a few times," said Djokovic.

"It's not much more to say than hope that she will be found, that she's okay. It's terrible … I can imagine just how her family feels that she's missing."

The Women's Tennis Association (WTA) has opened an investigation into Peng Shuai's sexual assault allegations against a former China leader.

Shuai, 35, 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.

In the post, published on Tuesday before subsequently being deleted, the 2013 Wimbledon doubles champion alleged that the pair had extramarital relations and she had developed feelings for him.

All of Peng's content has since been removed from Weibo and numerous reports suggest she has not been seen in recent days.

The WTA's chairman Steve Simon addressed the matter on Sunday in a statement that said: "The recent events in China concerning a WTA player, Peng Shuai, are of deep concern. As an organisation dedicated to women, we remain committed to the principles we were founded on – equality, opportunity and respect.

"Peng Shuai, and all women, deserve to be heard, not censored. Her accusation about the conduct of a former Chinese leader involving a sexual assault must be treated with the utmost seriousness.

"In all societies, the behaviour she alleges that took place needs to be investigated, not condoned or ignored. We commend Peng Shuai for her remarkable courage and strength in coming forward.

"Women around the world are finding their voices so injustices can be corrected.

"We expect this issue to be handled properly, meaning the allegations must be investigated fully, fairly, transparently and without censorship.

"Our absolute and unwavering priority is the health and safety of our players. We are speaking out so justice can be done."