Following Carlsen's withdrawal, Niemann admitted cheating previously in online chess during an interview with St Louis Chess Club commentator Alejandro Ramírez.
He denied that he had ever cheated in over-the-board chess.
Later that month, Carlsen again sparked controversy when he resigned in protest after just one move while playing a game against Niemann in the Champions Chess Tour: Julius Baer Generation Cup.
The incidents led to widespread speculation, including by Hikaru Nakamura — who reported on the controversy on his Twitch channel, that Carlsen suspected Niemann had been cheating.
Carlsen later confirmed that he indeed suspected Niemann of cheating in a statement issued on September 27, accusing Niemann of cheating "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. This game contributed to changing my perspective," he wrote.
The controversy deepened when Chess.com — the world's largest chess site — issued a 72-page report that alleged that Niemann 'likely cheated' over 100 times in online chess matches.
Chess.com is in negotiations to buy Carlsen's company Play Magnus Group but denied its decision to release the report was linked to the discussions.
Niemann has been critical of Nakamura's reporting on the scandal, writing on Twitter following his interview with Ramírez, "Hikaru has thoroughly enjoyed watching all of my interviews and enjoyed criticising every single detail and making frivolous implications. I'd like to see him watch my entire interview today and see what he has to say."
Niemann launched the civil law suit in the Eastern District of Missouri and his represented by The Gartner Law Firm.
He alleged that "Carlsen has exploited his notoriety and success in chess to create an incredibly lucrative global brand and online chess company, the Play Magnus Group."
"Carlsen, having solidified his position as the 'King of Chess,' believes that when it comes to chess, he can do whatever he wants and get away with it."
Niemann's statement of claim alleged that Carlsen "snapped" after his defeat to Niemann.
"... Carlsen viciously and maliciously retaliated against Niemann by falsely accusing Niemann, without any evidence, of somehow cheating during their in-person game..."
Niemann also claimed that chess.com acted in "collusion" with Carlsen and Play Magnus to "lend credence to Carlsen's unsubstantiated and defamatory accusations of cheating."
He also accused Nakamura of "amplifying and attempting to bolster Carlsen's false cheating allegations."
And the claim states that Chess.com CEO Danny Rensch "issued defamatory press releases, and leaked defamatory 'reports' to prominent press outlets."
The claim alleges that the "malicious defamation and unlawful collusion" had "destroyed Niemann's remarkable career in its prime and ruined his life."
Niemann is seeking NZ$176 million (US$100 million) in four causes of action — totalling $704.5 million in damages.
Niemann posted a link to the legal action on Twitter, writing "My lawsuit speaks for itself", in reference to an interview where he famously said, "The chess speaks for itself."
