I do think that today most tennis players are getting themselves as fit as possible because the competition is more fierce, the prize money is way up than ever before.
However that being said, tennis is still mostly a game of skill, and I'm sure the players of the past would have adapted to todays conditions.
Consider the fact that at 35 years old Andre Agassi was top 10 last year, at that age getting to the finals of the US open. He has beaten roddick and blake consistently, and has an equal record against Daydenko and Ljubicic.
That same agassi who has a lifetime record of 2-6 against Lendl, the same Lendl who has a 2-5 lifetime record against the dominating Borg, who has a 7-6 record against Ashe, who has a 2-12 record against Laver, etc etc.
If laver could beat ashe, who could beat Borg, who beat Lendl, who beat Agassi, who beats up on Roddick and blake, I really think the past greats are greatly underestimated against today's champs.