Tennis players ARE the best athletes. How many people can perform their sport in 100+ degree weather, on multiple different surfaces and with about 20+ muscularly memorized motions in split seconds, on a 95% basis? In basketball, football, baseball, soccer (on a smaller scale for goalies), hockey (with, again, smaller scale for goalies), if you did what you were trying to do and did it on a 95% basis, you would be GODS! 95% hitting in baseball is mind blowing for TEE BALL let alone MLB. 95% shooting is about the highest % in the NBA for free throw shooting and layups. In football, if you threw 95% completion percentage in a game, you would set an NFL record. So the fact that pro players can hit forehands and backhands generally with a 95% success percentage is amazing and under the circumstances they must do it in (balls coming at them very fast, sliding depending on the court, etc.) is amazing.
In terms of conditioning, it's not even close. No athlete really has to do what tennis players do, with maybe the exception of soccer players. In basketball, you play 40-48 minute games (and very rarely do basketball players play full games, I believe the highest mpg avg in the nba was in the low 40s last season) and you have timeouts, turnovers, quarterly and halftime breaks, tv breaks, foul breaks all to rest. Tennis players have a few seconds between serves and a couple minutes between change of court to take breaks. Some matches last under an hour for women's matches or an hour and a half in men's, but thats when someone is completely overmatched and it's not even really a competition anyways. Most pro matches are about 2+ hours with very little interruption in play. Also, you have to run full speed just about nonstop in tennis, whereas sports like basketball you can setup a half court offense and generally be able to stand around half of the possession or baseball where you're standing in place on defense 70% of the time or, in your team's batting portion of the inning, only hitting once every nine times and generally only hit every other inning, or in some cases, every three innings.
And how many athletes can claim they have to react to objects flying at them at over 100+ mph? Baseball hitters don't face pitchers that can throw 100 on a regular basis; there are very few people who can throw a baseball 100mph, but you can go anywhere and find someone with a 100mph serve.
So, in reality, we are the best athletes. When you play tennis, you're competing in sprints which together total a marathon. You must be able to complete more difficult athletic feats on a more consistent basis than any other athlete, and you must be better conditioned than any athlete, and those are the things that pretty much define athleticism.