The difference between a 5.0 and a 6.0 player is quite big, the difference between them and a pro in the top 100 is SUPER DUPER BIG. A pro is generally considered to be a 7.0 player, and even within the 7.0 the difference is pretty huge. I mean a guy ranked 600 in the world is technically 7.0 as is Marat Safin.
Yeah, they make it look easy and you can sit there and watch Safin and think to yourself "oh, you just have to this... and make sure you do that... I can't wait to go out on the court and apply what I've learned" but if you were to actually play against even a guy ranked 1001 in the world (and there might be 100 guy who are all co-ranked 1001) you'll see that he's a lot better than any 5.5 player you've ever met.
I once played a guy who had some ATP points back in the day. He was around 5'8" and didn't hit with a lot of power, and we had some really long points and games, but he beat me 6-1, 6-1. It took nearly an hour and 45 minutes, but he smoked me. He was basically just moving the ball corner to corner, only REALLY playing at deuce or if he was facing a break point. I could say "hey, I'm almost as good as him, all I have to do is improve X and Y" but I could also easily say "he beat me without REALLY trying... I didn't even see what he's truly capable of doing today."
If you're a 6.0 player with a huge forehand and a freakishly nasty serve, yeah, if you're in the zone one day it's concievable that you could take a game or two off almost anybody in the world if they're playing less than their best, but them's a lot of "if's."
A guy told me about Pete Sampras practicing with some UCLA players. This guy, Tobias Clemens, was ranked top 10 in the nation D-1, but this guy told me the weight of Sampras' shots were giving Clemens, who had to put everything into it, a hard time. And Sampras was a notoriously lazy practice player. If you're 5.0 or above, you can practice with anybody and make it somewhat worth their while. I know a guy I can beat who's played with Tursunov (I think he's ranked top 70 in ATP), and he said he didn't waste his time. But that's just practicing.
In a match you'd quickly see that the top 200 pros are simply playing a different game than us.