Arriving at an acceptable definition for "aggressive" may be the most difficult and contentious aspect of this article.
For our purposes, an aggressive player is one who is willing to put pressure on an opponent on every shot. Although the placement and speed of the shot play a part, more central to the idea of applying pressure is an ability to hurry the opponent. Typically, an opponent is hurried because an aggressive player takes the ball early or comes to net.
"Aggressive" is not a synonym for "effective," as many of the most effective players, such as Rafael Nadal and Novak Djokovic, exercise patience and use selective aggression to finish off points. Patience typically is not a virtue of the most aggressive players.
Furthermore, an aggressive player is not necessarily a hard-hitter, and a hard-hitter is not necessarily an aggressive player. John McEnroe, one of the most aggressive players in tennis history, was by no means a big hitter.
Prime examples from the past of what we consider aggressive players are Boris Becker, McEnroe, Pete Sampras, and, if you go way back, Jack Kramer.
Only one American made our top-10 list, although Mardy Fish and John Isner might rank 11th and 12th.