The Most Important Points

"Playing to the score - Every point in a match is naturally important but there are certain points in each game, certain games in each set, and certain sets in a match which are crucial. The player who knows those points and games, and makes a special effort to win them, will greatly increase his chances of victory. The crucial points in a game are the third and the fifth. The crucial games in a set are the fourth, particularly the seventh and the ninth. The crucial sets are the first, in a two-out-of-three match, and the third, in a three-out-of-five match."

-Bill Tilden, How to Play Better Tennis (1950) 
Chapter 16, "Maintaining Pressure on Your Opponent"