Why can't you win 6 5 in tennis?
If the score reaches 6–5 (or 5–6), one further game is played. If the leading player wins this game, the set is won 7–5 (or 5–7). If the trailing player wins the game, the score is tied at 6–6 and a special tiebreaker game is played. The winner of the tiebreak wins the set by a score of 7–6 (or 6–7).
If the score reaches 6–5 (or 5–6), one further game is played. If the leading player wins this game, the set is won 7–5 (or 5–7). If the trailing player wins the game, the score is tied at 6–6 and a special tiebreaker game is played. The winner of the tiebreak wins the set by a score of 7–6 (or 6–7).