from the sports I've played, physically the toughest sport has been soccer for me (never played waterpolo, so can't comment on it).... constant running - non stop... if you're playing at a competitive level, and you stop moving, you're hurting your team... you need to be a top physical shape to play soccer at a high level.... I never played at a high level, but even at the highschool level (where I only played during my sophomore year, and even that, it only lasted a few months), it was pretty demanding overall.
I started playing tennis at 6. Played in a lot of tournaments, and today (at over 30) still play from time to time casually.... tennis is definitely a physically demanding game... the comments in this thread about it being easy - well so is hockey if you're playing pond hockey with a bunch of buddies with no serious competition.
Playing tennis competitively requires good physical conditioning... lots of side to side and back and forth movement.... I've had shoulder problems, knee problems and experienced mental exhaustion a lot. It's a chess game out there, and every point makes you think what to do next, what your opponent is going to do, what you can do to throw your opponent off their game, what your opponent will throw at you next.
And these thoughts are different at every stage of the game. When you start a game, the thinking process is different... when it's 30-0 it's different, when it's 15-40, or 15-30, or deuce, it's all different.
Throw in playing a 2hr + match, and sometime longer than 3 or even 4 hrs, to all that, and it gets even harder... you're waiting for a serve trying to watch how your opponent will toss his serve, to see what kind of swing he will take, which tells you where the ball could land, and the pace behind it... then you have to worry about putting it in a place where they won't simply win the point with an easy winner.... when you're serving the thoughts are reversed - hit the corner of the box, with a certain spin - and not too much spin so you can still get the pace - and then try and predict the returners thought pattern of where they will put the ball.
There is a HUGE mental game involved in tennis. And that mental part alone takes a physical toll on you.... you don't have a bench to go to, where your coach can tell you what to do... you have to figure that out from figuring out how your opponent themselves think on the court.
most people here seem to really underrate the demand the sport has overall. I've played hockey as well... and while I'm a much bigger hockey fan than I am a tennis fan, I would say that 1 competitive tennis match takes more toll on a person overall - all things considered - than 1 full hockey game does... and in most tournaments you're playing every day for a couple weeks before you get your first break... and that doesn't include practicing after the games, for the next match.
