When the score reaches 40-40 in a game. From deuce, a player must win two consecutive points to win the game (first to advantage, then to game). In theory, a deuce game can go on forever. Some matches have had games with 10+ deuces. The server generally has the advantage at deuce because they're serving, but a deuce game on a pressure point? That's pure stress for both sides.
"Deuce for the fifth time this game. Neither player can string two points together."