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 did not receive a response.
Earlier this month, the Norwegian withdrew from the Sinquefield Cup in Saint Louis following his surprise defeat against American star Niemann — the first time he has withdrawn from a tournament in his career, according to chess24.
The 19-year-old admitted to cheating at the ages of 12 and 16 but said in an interview with the St. Louis Chess Club he had never cheated in over-the-board games.
“I’m saying my truth because I don’t want any misrepresentation,” said Niemann. “I am proud of myself that I have learned from that mistake, and now I have given everything to chess. I have sacrificed everything for chess.”
The tension between Niemann and Carlsen has rocked the chess community. Niemann said he had been removed from popular website Chess.com following Carlsen’s tweet and that “the entire social media and chess world is completely attacking me and undermining me.”
“To see my absolute hero (Carlsen) try to target, 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 an explanation and response with the hope of finding a resolution where Hans can again participate on Chess.com.”
Carlsen and Niemann played two further games against other opponents following the former’s sudden resignation on Monday. Carlsen is two points behind leader Arjun Erigaisi in the tournament standings after eight rounds, while Niemann is four points back.
“It just looks very odd now. Clearly, it all makes sense if, supposedly, Hans is cheating and he doesn’t want to play him, but if he isn’t (cheating), then it is really very wrong.
“So I don’t know, we have to see. Again, everybody is expecting some kind of big rabbit from the hat with Magnus, but he just doesn’t want to play Hans, it seems.”
Levon Aronian, who is also competing in the tournament, said Niemann “has not been the cleanest person when it comes to online chess,” but added that “this is a problem that requires a solution.”
CNN’s Ben Morse contributed to reporting.