Four-time Grand Slam champion Naomi Osaka said that she was in "shock" about fellow tennis star Peng Shuai, who has not been heard from since alleging that a powerful Chinese politician sexually assaulted her.

Osaka added her voice to growing concern within tennis about Peng's fate, with men's number one Novak Djokovic and numerous other players in recent days saying they were deeply worried about her.

The 24-year-old Osaka wrote a short statement on Twitter, where she has 1.1 million followers, accompanied by #WhereIsPengShuai -- a hashtag which has been widely used on social media.

"Censorship is never ok at any cost, I hope Peng Shuai and her family are safe and ok," the Japanese former world number one wrote.

"I'm in shock of the current situation and I'm sending love and light her way."

Pressure is growing on Chinese authorities to clarify the status of the 35-year-old Peng, a former Wimbledon and French Open doubles champion.

Peng alleged on the Twitter-like Weibo earlier this month that former vice-premier Zhang Gaoli had "forced" her into sex during a long-term on-off relationship.

It was the first time the #MeToo movement has struck at the top echelons of China's ruling Communist Party, but the post was swiftly deleted and nothing has been heard from Peng since.

The Women's Tennis Association called Sunday for Peng's claims to be "investigated fully, fairly, transparently and without censorship".

WTA chairman Steve Simon told The New York Times that they had information that she "is safe and not under any physical threat".

China has remained quiet about Peng and its national tennis association did not respond to AFP requests for comment.

Details of Peng's accusations have been scrubbed from China's heavily censored Internet.