Someone once said 'the more things change, the more they stay the same', and so it is in many ways with tennis technique as well.

Here are some modern examples.  In the 1990s to hear the talk on TV and the local courts one might have thought that the Williams sisters were the first ones to have ever hit an 'open stance' forehand.  Not hardly, although Venus and Serena may have re-defined power tennis.   One might think that Goran Ivanisavic or Andy Roddick invented the cannonball serve.  No way.  Even if you forget Roscoe Tanner of a few decades ago, there was Lew Hoad and Jack Kramer before that. 

A history of hard servers, yes, but in the earliest days (19th century) of Lawn Tennis there was no 'over hand' serve, so there is a real change.  By the late 1920s Bill Tilden's precise and powerful service was legendary - showman he was, it is said he would sometimes hold 4 balls in one hand to start a game and finish with a win 4 serves later.  

(You can't 'double fault' if you only have one serve, which is a rules change we won't discuss here).

Another debate we will not enter is that of the greatest player of all time.  There have been great players in each generation, but something that has changed is the training and science that goes with it that allows more maximization of natural potential.

...to be continued...