Magnus Carlsen of Norway retained his world championship on Friday in Dubai after his challenger, the Russian grandmaster Ian Nepomniachtchi, committed the last of a series of blunders that turned their once-taut match into a relatively easy victory.

Carlsen’s victory came in the 11th game of an event that had been scheduled to last 14 games. The final score was 7.5 to 3.5 points, with each victory worth one point and draws worth half a point.

The loss in the final game by Nepomniachtchi completed one of the worst collapses in a title match in chess history. After the first five games ended in draws, Nepomniachtchi lost four of the last six games. His last three defeats were mostly the result of self-inflicted wounds, as Nepomniachtchi made critical and relatively simple errors in each of them.

In the final game, the players’ chances were equal until Nepomniachtchi made a rash decision to advance the pawns in front of his king to attack one of Carlsen’s rooks, exposing him to a counterattack. Carlsen actually missed the best continuation, which would have forced Nepomniachtchi to give up his queen to avoid being checkmated, but still steered his way to a rook-and-pawn ending where he had a considerable advantage.

Carlsen eventually promoted one of his pawns to a queen and Nepomniachtchi resigned soon afterward.

The turning point in the match was Game 6, an epic struggle of 136 moves won by Carlsen that lasted 7 hours 45 minutes. It was the longest game in world championship history and clearly took a physical and psychological toll on both players, though it was harder on Nepomniachtchi.

After retaining his title, Carlsen pointed to Game 6 as the critical moment. “That sort of laid the foundation,” he said. “The final score was probably a bit more lopsided than it could have been.”

Nepomniachtchi acknowledged his collapse was unprecedented. “OK, I have lost some stupid games, but never so many in such a short time,” he said, adding that he did not know what had gone wrong. “If I knew, I would do something about it.”

Carlsen collected $1.2 million for his victory and Nepomniachtchi took home $800,000.

Carlsen compared his victory this year to 2013, when he beat Viswanathan Anand of India to become the world champion. The winning margin for that match, a best-of-12, was similar, 6.5 to 3.5.

This was Carlsen’s fifth victory in a world championship match. He has now held the title for eight years, longer than his predecessor, Anand, who was champion from 2007 to 2013, but only a bit more than half the time of Garry Kasparov, the modern record-holder, who held the title from 1985 to 2000.

Carlsen seemed drained and not overly elated after winning the last game. He said, “It is hard to feel that great joy when the situation was so comfortable to begin with.”

As the runner-up in the title match, Nepomniachtchi is already seeded into the next candidates tournament, which will have eight players, to select the next challenger for the world championship. Asked if his experience in the title match would help him, he said he hoped it would, before adding, “Experience is never easy.”