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 reached out to Carlsen’s representatives for comment but has not received a response.
Earlier this month, the Norwegian withdrew from the Sinquefield Cup in Saint Louis after his surprise loss to American star Niemann – the first time he has withdrawn from a tournament in his career, according to chess24.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 ages 12 and 16, but said in an interview with the St. Louis Chess Club he never cheated in offside games.
“I tell 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 rocked the chess community. Niemann said he was removed from the popular Chess.com website following Carlsen’s tweet and “everyone in social media and chess is completely attacking and undermining me.”
“To see my absolute hero (Carlsen) trying to target, trying to ruin my reputation, ruining my chess career and doing 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 invited Hans to provide an explanation and response in hopes of finding a solution to allow Hans to participate again on Chess.com.”
Carlsen and Niemann played two more games against other opponents after 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.
“It looks very strange now. Obviously it all makes sense if, supposedly, Hans is cheating and he doesn’t want to play him, but if he’s not cheating then it’s really very wrong.
“So I don’t know, we have to see. Again, everyone is expecting some sort of big hat-rabbit with Magnus, but he just doesn’t want to play Hans, it seems.”
Levon Aronian, who is also taking part in the tournament, said Niemann “isn’t the cleanest person when it comes to online chess”, but added that “this is a problem that needs fixing”.
CNN’s Ben Morse contributed reporting.