Novak Djokovic has spoken of his "shock" at the disappearance of Peng Shuai as the ATP joined calls that her claims of sexual assault be investigated.
Peng, who accused a retired Chinese Communist Party official of the abuse on November 2, has not been seen or heard of since last week, prompting fears over her safety. "I did hear about it a week ago and, honestly, it's shocking." said Djokovic. "More so that it's someone that I've seen on the tour in previous years quite a few times."
Djokovic's intervention comes after the ATP joined the WTA in calling for a "full, fair and transparent investigation" – although stopping short of threatening to boycott staging events in China.
The men's tour had been under pressure to follow the lead of its women's counterpart, which warned on Sunday it could pull its tournaments from the country unless there was such an inquiry.
Andrea Gaudenzi, ATP chairman, said: "There is nothing more important to us than the safety of our tennis community.
"We have been deeply concerned by the uncertainty surrounding the immediate safety and whereabouts of WTA player Peng Shuai. We are encouraged by the recent assurances received by WTA that she is safe and accounted for and will continue to monitor the situation closely.
"Separately, we stand in full support of WTA's call for a full, fair and transparent investigation into allegations of sexual assault against Peng Shuai."
The International Olympic Committee, International Tennis Federation and ATP are under major pressure to join the WTA's boycott threat.
Julian Knight MP, the chairman of parliament's Digital, Culture, Media & Sport select committee, told Telegraph Sport: "I applaud the stance the WTA is taking in this respect.
"These allegations need to be fully investigated and this individual needs to be produced.
"China has a very dark history when it comes to human rights and this gives the whole sporting world more than pause for thought."
He added: "Frankly, all avenues should be explored in terms of applying pressure on China to ensure this young lady is out in public once again.
"I would not leave anything off the table and the WTA's stance is one that I thoroughly support."
Knight stopped short of calling for British sensation Emma Raducanu – who became a social media star in China after conducting interviews in fluent Mandarin following her fairy-tale US Open triumph – to issue an appeal on behalf of Peng.
Telegraph Sport has approached the IOC, ITF, and representatives of Raducanu for comment.