SEOUL—A simple computer pop-up message gave humans some relief in a high-profile man-versus-machine board-game battle: “AlphaGo resigns.”

Those words on Sunday allowed South Korean Go grandmaster Lee Se-dol to claim his first victory in a five-game match against AlphaGo, an artificial-intelligence project developed by Alphabet Inc.’s Google. The win in the ancient chess-like game, after three straight losses, restored some pride to Mr. Lee—as well as big smile across his face.

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