An allegation of sexual assault by tennis star Peng Shuai and its aftermath have brought the realities of China's censorship and secrecy around its leadership to global attention just as Beijing prepares to host the Olympics.

Peng, who was not heard from publicly for nearly three weeks after alleging former vice premier Zhang Gaoli sexually assaulted her, told International Olympic Committee president Thomas Bach on Sunday by video call that she was safe and well.

The Women's Tennis Association (WTA), which has led calls for an investigation into her allegations, remained unconvinced. Some Western governments and top current and former tennis players have called for reassurance that Peng is safe.