Tennis is somewhat unique among sports, in my opinion, in that it may be the sport that MOST combines great athleticism as well as great amounts of skill that must be honed for many years from a young age....
It doesn't matter how fast or how strong you are on a tennis court, if you can't for example, follow up a blazing forehand with a soft shot into the open court once in a while for a winner.
This is brought up during these discussions all the time. By the way, I love these discussions.
But yeah, somebody brings up how skill intensive tennis is, and it's true. Off the top of my head, only world class gymnasts and figure skaters start systematically training as early and often under expert guidance the way tennis players do.
Still, with all else being equal, the better athlete wins. Look at Borg's dominance on grass and clay. Was he REALLY the most talented ball striker on the tour for those years?
Far as blazing speed goes, 90% of the time spent on court it probably doesn't PHYSICALLY matter. During the rally, you don't need blazing speed to stay in it. Just ask David Nalbandian's stomach. But at the same time, I wrote "physically" because I remember during his prime, Chang would really make his opponents hit 2 or 3 extra shots to win the point, and by the later sets, they're making unusual unforced errors left and right because they're so spooked by Chang's defense. Speed takes a mental toll on your opponent.
Athleticism is a huge asset in pro tennis where everybody can hit the ball well.
I think people who bring up the "tennis requires skill" overstate its importance because they're mostly framing their argument within the context of recreational tennis where there are way more mismatches. So yes, on public courts, you see a 230 pound former college player beat up on a ultra fit 20 year old triathlete and you think "skill is more important." But you get a 170 pound former college player and put him up against the 230 pounder and you may suddenly think "wow, being fit and being able to move is really important."
On the ATP tour, less of these mismatches. Everybody has great technique, good footwork, etc. Yes, the top players have an extra gear, but they can all hit the ball, and there is almost always some overlap between the players, meaning a guy ranked 90 in the world, if he's playing out of his mind at 100% and he comes up against a dour Andy Murray playing at 85%, he's got a shot against the number 4 player in the world. At that level, it's about being mentally stronger, better at taking negative feelings and channeling them in positive ways, and being a fitter, better athlete.
All else being equal, athleticism makes all the difference in the world. If only for those 2 or 3 points that really determine the outcome of a set, having that extra athletic gear, being able to run down that extra ball, makes the difference. Sampras did this a lot. He would look almost disinterested for most of the set, and then all of the sudden at 3-3 or 4-4 he would hit a running forehand, break serve, and coast out the set. He was one of the best athletes in tennis at the time, but he only needed the athleticism for a few points during the set.
Just think about it, if anybody in the top 10 gained 15 pounds of fat, how much do you think it would affect their ranking? Extra fat diminishes your athletic ability, that's all it does. Makes you less agile, less fast, and less explosive. Verdasco, Agassi, and Ginepri are three great examples of players who reached a new level just by becoming more fit (fit=maximizing and enhancing your natural athleticism). It matters a lot.