Tennis greats Roger Federer and Rafael Nadal have voiced concern for missing Chinese player Peng Shuai, as advocates call on Olympics officials to push China for proof of her safety.

Ms Peng, a two-time grand slam champion and former number 1 in doubles, disappeared after making allegations of sexual assault more than two weeks ago against a former prominent Chinese government official.

Chinese state media employees have since posted images and video of the tennis star to Twitter, which they claimed were filmed on Saturday.

Another video was posted on Twitter by the editor of the Global Times, Hu Xijin, purporting to show Ms Peng at a youth tennis championship in Beijing on Sunday. 

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The footage and images have been met with widespread scepticism on social media, along with demands for Ms Peng to appear in public in person.

Speaking on Italian television, 20-time grand slam winner Federer joined the chorus of human rights organisations, sporting bodies and athletes in voicing his concern for Ms Peng.

"She's one of our tennis champions, a former world number 1, and clearly it's concerning," Federer told Sky Italia.

"I hope she's safe. The tennis family sticks together and I've always told my children as well that the tennis family is my second family. I've been on tour for 20-25 years and I love the tour, I love the people that are there, [they] are special, the players as well, and she's one of them."

Nadal told French newspaper L'Equipe: "The most important thing is to find out whether she is OK. All of us in the tennis family hope to see her back with us soon."

Tennis players have been emboldened by the unequivocal support of the Women's Tennis Association and its chairman and CEO Steve Simon, who has threatened to pull the WTA's events out of China. There are almost a dozen such events next year, including the WTA final.

Liz Throssell, a spokeswoman for the UN human rights office in Geneva, said it was calling for "an investigation with full transparency into her allegation of sexual assault".

Athlete advocacy group, Global Athlete, has asked the International Olympic Committee (IOC) to suspend the Chinese Olympic Committee until Ms Peng's safety is guaranteed.

"The IOC must use its substantial leverage to ensure that the international community is provided proof of Peng's whereabouts," Global Athlete head Rob Koehler said.

'It's embarrassing not to speak out'

China is just two-and-a-half months from hosting the Beijing Winter Olympics, which is already facing a diplomatic boycott over allegations of crimes against humanity involving at least one million Uyghur Muslims and other ethnic minorities.

Emma Terho, the newly elected head of an IOC's Athletes' Commission, issued a brief comment on Saturday saying "we support the quiet diplomacy" approach favoured by the IOC.

"Experience shows that quiet diplomacy offers the best opportunity to find a solution for questions of such nature," the IOC had said in an earlier statement.

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

It also said it has received assurances that the former Olympian is "safe".

French tennis player Nicolas Mahut, who said he won't play in China if the situation with Ms Peng isn't resolved, said the IOC needed to do more.

"It's really embarrassing to not speak out. They should say something a few months before [the] Olympics in China," Mahut said.

"I hope they will do something soon."

AP/ABC