Norway's Magnus Carlsen announced his withdrawal after three rounds of the Sinquefield Cup ©Getty Images

Norwegian world chess champion Magnus Carlsen has withdrawn from the Sinquefield Cup in the United States, following his surprise defeat to home favourite Hans Niemann.

Carlsen was playing with the white pieces when he lost his 53-game unbeaten run in the classical discipline to Niemann, the lowest ranked player in the tournament.

He announced his withdrawal from the Sinquefield Cup in St Louis after three rounds on Twitter, also including a link to a video clip of Portuguese football manager José Mourinho saying "I prefer not to speak, if I speak I am in big trouble".

"I've withdrawn from the tournament," Carlsen said.

"I've always enjoyed playing [at the Saint Louis Chess Club], and hope to be back in the future."

Carlsen's results from the tournament have been annulled.

His decision prompted speculation of foul play for his opponent, with American player Hikaru Nakamura expressing his belief that it represent a form of protest.

"I think that Magnus believes that Hans probably is cheating," Nakamura said, as reported by ABC.

"You really have no way of proving that, if you really believe that, so I think Magnus' way of basically saying that is he's not going to actually say it, but he's putting out the stuff saying 'if I talk I'm in trouble', and he's withdrawing to make the point without publicly making the point."

However, other players have dismissed those suggestions, with Uzbekistan's Rustam Kasimdzhanov telling chess24 "if this is based on the game alone, then this sounds like a most frivolous accusation".

Nineteen-year-old Niemann drew with France's Alireza Firouzja in the fourth round, and admitted he was surprised by Carlsen's withdrawal.

"I'm speechless to be honest," he said, as reported by ABC.

"It's very weird.

"I don't want to draw any conclusions.

"It's very strange, but at least I got to beat him before he left."

A 15-minute broadcast delay and increased radio-frequency identification checks have been among the measures implemented by the Grand Chess Tour, at the request of anti-cheating arbiter David Sedgwick.

Uzbekistan's Rustam Kasimdzhanov, right, dismissed suggestions that Norway's Magnus Carlsen, left, had withdrawn because of suspected cheating as
Uzbekistan's Rustam Kasimdzhanov, right, dismissed suggestions that Norway's Magnus Carlsen, left, had withdrawn because of suspected cheating as "a most frivolous accusation" ©Getty Images

Saint Louis Chess Club executive director Tony Rich said it respected Carlsen's decision.

"A player’s decision to withdraw from a tournament is a personal decision, and we respect Magnus' choice," Rich commented.

"We look forward to hosting Magnus at a future event in Saint Louis."

The Sinquefield Cup has a $350,000 (£300,000/€350,000) total prize fund, and is the final leg of the Grand Chess Tour.

Carlsen has been world chess champion since 2013, winning the title five times.

However, he announced in July that he will not defend his crown at next year's World Chess Championship.