The pair were playing in the Julius Baer Generation Cup when Carlsen turned off his screen and left the match without explanation – the latest twist in an apparent feud between the two players.
CNN has contacted Carlsen’s representatives for comment, but received no response.
Earlier this month, the Norwegian withdrew from the Sinquefield Cup in Saint Louis after his surprise defeat to American star Niemann – the first time he withdrew from a tournament in his career, according to chess24.
The 19-year-old admitted to cheating when he was 12 and 16 years old, but said in an interview with the St. Louis Chess Club that he had never cheated in over-the-board games.
“I’m telling my truth because I don’t want any misrepresentation,” Niemann said. “I’m proud of myself for learning from that mistake, and now I’ve given everything to chess. I’ve sacrificed everything for chess.”
The tension between Niemann and Carlsen has turned the chess community upside down. Niemann said he was removed from the popular Chess.com website after Carlsen’s tweet and that “the whole social media and chess world is completely attacking and undermining me.”
“To see my absolute hero (Carlsen) try to aim, try to ruin my reputation, ruin my chess career and to do it in such a frivolous way is really, really disappointing,” he added.
Neither Niemann nor Chess.com responded to CNN’s request for comment.
Rensch continued: “We have invited Hans to provide explanations and answers in hopes of finding a solution that Hans can rejoin on Chess.com.”
Carlsen and Niemann played two more games against other opponents after the sudden resignation of the first on Monday. Carlsen is two points behind leader Arjun Erigaisi in the tournament standings after eight rounds, while Niemann is four points behind.
“It just looks really strange now. Obviously it all makes sense if Hans is supposedly cheating and he doesn’t want to play with him, but if he doesn’t (cheating) then it’s really very wrong.
“So I don’t know, we’ll have to see. Again, everyone expects some kind of big rabbit from the hat with Magnus, but he just doesn’t want to play Hans, it seems.”
Levon Aronian, who also participates in the tournament, said Niemann “hasn’t been the cleanest person when it comes to online chess,” but added that “this is a problem that needs a solution.”
CNN’s Ben Morse contributed to the coverage.