The Bollettieri era, through Agassi and to a lesser extent, Jimmy Arias, ushered in the ripped forehand (Bollettieri's 'killer forehand' so-called) where winning points could now be ripped from the backcourt. If you look at the videos pre late 80's, this was rarely managed even though they had by then the graphite racquets to do this. The received wisdom was that you couldn't or didn't hit through the court to make points but Agassi showed you could. It then became a routine play in Agassi's game style.
Since then it's spread like wildfire through the pro ranks and upper levels of tennis. No doubt all kinds of factors have converged to cement this predominant game style but it seems to me that this has now displaced all other game styles and largely made them redundant. Federer himself possessed a serve volley game style at the start of his career (look at the 2001 Sampras/Federer Wimbledon video) but very quickly moved away from that to rely on a backcourt-dominant game although tempered in his case by use of a fantastic volleying technique. Simply put, pro tennis is less reliant on technical repertoire. Some players can barely volley and so hardly ever venture to the net other than to put the ball away. Agassi volleying was a thinly-disguised joke.
My thesis is that this style of play - baseline bashing - lends itself particularly well to all the advantages that doping confers in that a player knowing he's secure on his legs can stay on the baseline and get behind virtually anything an opponent can throw at him. This isn't to say players aren't also superbly conditioned, trained or that they're able to forget good technique. What I'm saying is that doping enables tennis players to reach the top and stay there with a fairly limited technical range in their repertoire.
And you'll also note that all pros talk about these days is improvement forged through conditioning; Murray with his muscles and Djokovic with his magic diet and so on. They rarely talk of little technical improvements in their game and this in perhaps the most technical of all sports. The culmination of all this is perhaps best observed in the woman's game where virtually no gamestyle exists other than the baseline basher and where, in my view, even more evidence for widespread doping exists, circumstantially at least, than in the men's game. It is significant to note that the women signed up to WADA/ITF anti-doping a full year after the men, before which the players own organisation laughingly did all its own testing. Call me a cynic but there's more than an arguable case for saying the ITF is fiddling while Rome burns.