[5] In September 2022, Niemann became embroiled in a controversy after defeating reigning World Chess Champion Magnus Carlsen in the third round of the 2022 Sinquefield Cup.

In response, Niemann filed a $100 million defamation lawsuit against Carlsen, Chess.com, and fellow grandmaster Hikaru Nakamura.

[42][43] From November 24-December 3, Niemann participated in the World Youth Under-16 Olympiad in Konya, Turkey—scoring six consecuting wins before finally finishing in third place overall.

[3][48] Then in June, he won the inaugural ChessKid Games hosted by Chess.com in a duos team with Alexander Wang, with Niemann scoring 20 straight victories in the event and qualifying for the 2020 Junior Speed Chess Championship.

[3] In October, he participated in the 2019 World Youth Championships, leading the Open Under-16 section for seven rounds with a performance rating of nearly 2600 before ultimately finishing in ninth place.

[55][56][57] In April 2020, Niemann participated in the FIDE World Youth Championship, placing sixth in the American Continental Selection Open.

[60] In December, Niemann won the blitz competition at the VII Sunway Sitges International Chess Festival.

[67][68] In July, Niemann won the World Open in Philadelphia, Pennsylvania, defeating John M. Burke in tiebreaks.

[74] In August, he participated in the FTX Crypto Cup, winning a game with the black pieces in a second-round match against Magnus Carlsen.

[81][82] In April 2023, Niemann finished sixth out of 200 at the Menorca Open, with the top ten players all tying for first place with scores of 7/9 after the nine-round round-robin.

[88] International master Prraneeth Vuppala achieved a surprise victory against Niemann in the tournament, crossing the 2500 Elo mark necessary to become a grandmaster as a result.

[91] In June, Niemann won the 1000GM 2023 Las Vegas Super Swiss after tiebreaks with grandmaster Mikhail Antipov.

"[103][104] Niemann also played in the Djerba Chess Masters in February, defeating grandmasters Aryan Tari and Bilel Bellahcene but suffering another loss against Marc Andria Maurizzi.

[105][106] In April, he participated in the Grenke Chess Open, scoring 8/9 and coming in first place with victories against grandmasters Frederik Svane and Velimir Ivić.

[107][108] Then in May, Niemann played in the Dubai Global Police Chess Challenge, starting off slowly with three draws against 2400-rated international masters.

[109][110] In June, Niemann launched his official website, "GMHans.com," offering chess training, Q&A sessions, and personal advice.

[111] He also played for Tremblay-en-France in the French Team Championship, scoring a personal 5/7 with victories against grandmasters Markus Ragger, Arturs Neikšāns, and Marc Andria Maurizzi.

The event saw Niemann match up against several top-50 players, including Anish Giri, Nikita Vitiugov, and Étienne Bacrot, facing each of them in their respective FIDE countries (The Netherlands, England, and France).

As a result of his victories in the event, Niemann gained 22 rating points and entered the top 20 in the world in classical.

[120] From August to September, Niemann participated in the Speed Chess Championship, defeating grandmaster Maxime Vachier-Lagrave in the first round 12½-11½.

[122] The match was highly anticipated, with Niemann expressing a desire to take revenge on Carlsen for allegedly defaming him.

[126] Niemann has competed in the PRO Chess League since 2017, playing for the Las Vegas Desert Rats (2017), Saint Louis Arch Bishops (2019, winning team) and Norway Gnomes (2020).

[132] In the third round of the 2022 Sinquefield Cup, Niemann defeated then-World Chess Champion Magnus Carlsen with the black pieces, playing the Nimzo-Indian Defence.

[136] Later that month, on September 22, Niemann faced Carlsen again in a much-anticipated game during the Julius Baer Generation Cup.

[145] Many chess players and public figures commented on the controversy, including former world champion Garry Kasparov.

[146] The executive director of the St. Louis Chess Club, Tony Rich, said that no formal complaint was made in writing.

[147] Chief Arbiter Chris Bird published a statement affirming there was "no indication that any player has been playing unfairly" during the tournament but did not address the reason for additional security measures added after Carlsen's withdrawal.

[8][150] Chess.com's chief chess officer, Danny Rensch, confirmed that Niemann would remain suspended pending an explanation of his past cheating on the platform.

[157] On October 20, 2022, Niemann filed a $100 million defamation lawsuit against Carlsen, Chess.com, Play Magnus Group, and Nakamura regarding the cheating claims.

[159] By December 7, 2022, all defendants except Play Magnus Group had filed motions to dismiss on various grounds, including lack of jurisdiction and the State of Connecticut's anti-SLAPP laws.