Sherman's winning board. Photo courtesy of Sam Moch and Sam Kantimathi.

It’s the Scrabble equivalent of the four-minute mile: Mark Landsberg’s tournament score of 770, set at a 1993 tournament in Eagle Rock, Calif. For 18 years, no one came close to the mark in the pressure cooker of a tournament game, where top-ranked players face only solid competition. In recent years, a couple of club players topped the mark, leading Scrabble watchers to believe it could be broken in a tournament game someday.

That day came on Friday at the Northeast Scrabble Tournament in Stamford, Conn. Landsberg’s record ...