Tennis has evolved dramatically in the last 30 years.
The top 4 (Novak, Federer, Nadal, Murray) would wipe the floor with any of the previous decades champs (apart from perhaps Sampras)
They're just much better athletes these days. Science and Nutrition/proper training has pushed the level to what it is today.
A guy Like Agassi is just a lesser version of the current crop. he didn't have the power or the speed around court, not to mention the ability to grind out 5 sets at high intensity like the guys today have.
You go back further....Mcenroe, Becker, etc.... these guys wouldn't stand a chance. Mcenroe with todays training wouldn't be a top player. He's undersized and doesn't have the genetics in terms of speed and power.
Certain sports are hard to compare in terms of how athletes of decades past would compare with today..... Would Ali Beat Tyson? etc....it's all speculation....
Tennis is not like that. The guys today are simply better.