To Wear the Sudoku Crown, One Must Solve Any Number of Puzzles

The Dai-Vunk rematch was highly anticipated, yet Kota Morinishi, 34, from Tokyo, a four-time world champion who works in information technology, took an early lead, fueled by a sack of candy ever on offer from his team captain.

Ms. Dai had a rough start: In Round 2 of 10 she made mistakes on, or “broke,” the same puzzle three times; ultimately she erased the whole thing and restarted. In Round 3, while focused on fixing two broken puzzles, she forgot a puzzle and didn’t complete it before time was up.

Mr. Vunk finished Round 3 with three minutes to spare — “Could’ve been better,” he said — putting him in first place, with Ms. Dai second.

Byron Calver, 38, a civil servant in Toronto who sat next to Ms. Dai, was not thrilled with his showing. (His best finish was fifth, in 2010, but he had practiced too hard and burned himself out, he said. Now, after a hiatus, he was trying recapture what he had lost: “Discovering your mortality by being bad at Sudoku, the Byron Calver story,” he said.) When asked how Round 4 had gone, he said, “It did not go.” It involved Sudokus with arithmetic constraints. “I did great at the math, I just forgot how to do Sudoku,” he said.

And at least once that day, in desperation, Mr. Calver resorted to a “wild bifurcation” — “bifurcation” being Sudoku-speak for “guess.” Typically, it is a calculated trial-and-error guess, exploring one of two clear paths presented by a partially completed puzzle. But in such an either-or gambit, only one path is correct. Mr. Calver’s bifurcation was more reckless, he said, “insofar as it was spurred more from blind hope in the absence of a clear path forward than from any well-grounded expectation that progress would result.”