American great Serena Williams says the case of Chinese player Peng Shuai "must be investigated" and urged people "not to stay silent" about her welfare.
Peng, 35, has not been heard from since she made sexual assault allegations against a leading Chinese government official two weeks ago.
Chinese state media has released an email attributed to Peng, but doubt has been cast about its authenticity.
"I hope she is safe and found as soon as possible," said Williams.
Concern for Peng's welfare has continued to grow since she said she was "forced" into a sexual relationship with former China vice-premier Zhang Gaoli.