The WTA had called on China to investigate Peng's claims on Sunday, while also demanding an end to censorship of the former top-ranked doubles player.
Peng, one of China's biggest sporting stars, alleged on her Weibo social media account on 2 November that Zhang Gaoli, who used to be a member of the Politburo Standing Committee - China's top decision-making body - coerced her into sex and they later had an on-off consensual relationship.
Peng, 35, said in the post, which was deleted about half an hour after it was published, that she could provide no evidence to back her allegations.
"There is nothing more important to us than the safety of our tennis community," ATP chairman Andrea Gaudenzi said in a statement.
"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 internet in China is heavily censored and the private lives of top leaders are an especially sensitive subject. Zhang, now 75, was a vice premier between 2013 and 2018 and served on the Politburo Standing Committee between 2012 and 2017.
Concerns among the global tennis community have grown as Peng, the first Chinese player to top the world rankings when she was doubles number one in 2014, has not been seen since the post.
Men's world number one Novak Djokovic said he was shocked by the incident.
"I did hear about it a week ago. Honestly, it's shocking that she's missing, more so that it's someone that I have seen on the tour in the previous years quite a few times," the Serb said after his win over Casper Ruud at the ATP Finals.
"It's not much more to say than hope that she will be found, that she's okay. It's terrible... I can imagine just how her family feels that she's missing."
In a statement on Sunday, WTA chairman and CEO Steve Simon said that the recent events were of deep concern.
"Peng Shuai, and all women, deserve to be heard, not censored. Her accusation about the conduct of a former Chinese leader involving a sexual assault must be treated with the utmost seriousness," he said.
China's State Council Information office and the Chinese Tennis Association did not immediately respond to requests for comment on the WTA's statement.
The International Tennis Federation also did not immediately respond to requests for comment.
Simon told the New York Times on Sunday that no one at the Tour has talked directly to Peng but that he had received assurances from the Chinese Tennis Association that she was safe "and not under any physical threat."