Never has a chess world championship titleholder seemed as vulnerable.

On Monday, Ding Liren, the reigning world champion, will begin a match in Singapore to defend the title against the 18-year-old player Gukesh Dommaraju. According to many of the world’s best players, and Ding himself, he will be the underdog.

An absent and possibly ‘broken’ champion.

Ding, 32, from China, won the title in April 2023 by beating Ian Nepomniachtchi in a thrilling playoff. Afterward, amid physical and mental exhaustion, Ding vanished from the circuit for the rest of the year, withdrawing from many events that he had committed to playing. His absence was unusual for a champion.

When he did return to action, he was clearly not the same player. It was not only a problem of being rusty: He played tentatively and struggled in most of his games and seemed to lack confidence. His results were far below his previous level and also below the level of what might be expected of a world champion.

At a tournament in the Netherlands in January, he finished ninth out of 14 competitors. A month later, in a Chess 960 tournament in Germany, he lost 10 of 13 games. (Chess 960 is a variant in which the pieces on the back ranks are shuffled into one of 960 possible starting configurations.)

At a tournament in Norway in July, he finished last. At the Sinquefield Cup in St. Louis in August, he finished tied for second-to-last. And at the biennial Chess Olympiad in Budapest in September, he failed to win a game while playing the top board for his team.

As his performance suffered, Ding’s world ranking fell to No. 23 from No. 3.

“The question is whether he is sort of permanently broken from the last world championship that he played,” said Magnus Carlsen, the former world champion, on the podcast Chess Chat. “I’m not sure, but I think there is a possibility that he could be.”