The 19-year-old chess grandmaster says the world champion has mounted a libelous campaign against him since Niemann defeated Carlsen in September.
Niemann, a grandmaster, is competing in the U.S. Chess Championships tournament that began Wednesday in St. Louis.
World champion Magnus Carlsen abruptly resigned after making a single move in his highly anticipated rematch with Hans Niemann. Calls have increased for an investigation.
Carlsen, a 31-year-old Norwegian native, was expected to have a rematch next year with Ian Nepomniachtchi, the Russian who he defeated in the 2021 world championship.
"I'm just really happy," Rameshbabu Praggnanandhaa said. He is the youngest person to defeat Carlsen since the Norwegian became world champion.