The Norwegian chess grandmaster took to social media on Sunday to put out a statement regarding his withdrawal from multiple events recently and the reasons behind them.
The 31-year-old resigned from the Julius Baer Generation Cup last week after one move against Niemann. He also withdrew from the Sinquefield Cup in St. Louis, Missouri earlier this month after registering a surprise defeat to the 19-year-old.
"I believe that Niemann has cheated more - and more recently - than he has publicly admitted. His over-the-board progress has been unusual, and throughout our game, in the Sinquefield Cup I had the impression that he wasn't tense or even fully concentrating on the game in critical positions while outplaying me as black in a way I think only a handful of players can do," he said.
Martina Navratilova, who is always active on social media, reacted to Carlsen's statement by calling it a "serious accusation" that could lead to a "scandal."
"This is a serious accusation and if true quite the scandal, yikes," she tweeted.
Martina Navratilova lashes out at ACLU for their stance on transgender athletes
One topic Martina Navratilova has always been vocal about is the inclusion of transgender athletes in sports. While many are for it, the 65-year-old is completely against it.
On Sunday, she took shots at the American Civil Liberties Union (ACLU) for its stance on transgender people competing in sports. The ACLU is a nonprofit organization founded in 1920 to "defend and preserve" the individual rights and liberties of American citizens.
"Well, the ACLU, thanks mainly to Chase Strangio, has really gone sideways when it comes to sports and trans women competing- they will only accept full inclusion for trans women to compete, no mitigation, only self ID necessary. How crazy is that? Yup, this crazy," Navratilova said.
Chase Strangio, the person she said was responsible for it all, is a lawyer and transgender rights activist who also serves as the Deputy Director for Transgender Justice and a staff attorney with the ACLU.
The 18-time Grand Slam champion's stance on the issue has led to her being accused of bigotry, something she disagrees with.
"Not cool to be asking for the right of women to have our own spaces? That would be bigotry? What is not cool is not having those spaces… check your male privilege, please," she said.
"So let me ask you- you think it is wrong that biological women who are in a rape survivor group insist on having only biological females in the group, including the actual counselor? Or are they wrong to exclude trans women from that particular group?" she added.