In the world of tennis, it all comes down to what the court's surface is made of.
This is a quick thought experiment into why the French Open's court surface makes it the most psychically (and psychologically) demanding of all the major Grand Slam tournaments.
So, let's profile some court surfaces, shall we? In tennis, this means looking at the hard court, grass court and finally the clay court surfaces. To do this, I like to think each surface can be graded on 3 criteria: Play, Bounce, and Favored Hit Style.
The 'play' profile summarizes how quick or slow the ball tends to travel, and the overall game pace. 'Bounce' will cover typical ball trajectories, and how players expect to approach counter strikes. And 'favored hit style' details what hit strategy tends to work best year after year, on any given court surface.
The Hard Court, U.S. Open & Australian Open's surface.
Play -- Medium-fast to fast, very fast rebound speed
Bounce -- High bounce, big opportunity to apply spin to the ball
Favored hit style -- Flat Ball
Added all together, this makes this court surface likely the most ideal for tennis pros. High bounce gives a player the maximum amount of options for ball placement. Lob, passing shot, volleys, and every kind of effective spin out there all work well on this surface, and tend to compliment strong servers, skilled at intelligent return game. Matches go quickly here; no need to "grind it out" if you are truly a superior player.
The Grass Court, the surface at Wimbledon.
Play -- very fast, inconsistent ball placement, speed and power rewarded
Bounce -- tend to be low, skidding and fast, rarely rising above knee height
Favored hit style -- Serve and Volley
For Wimbledon and grass, play is yes, more inconsistent. You are dealing with grass after all. But similar to the hard court, matches tend to go fairly quick as grass still plays "very fast". And low, don't forget that. So if you have a hell of serve and volley game, that's paramount here. All else required is speed burst, and a strong(er) core; you'll be returning a solid amount of 'line drives'. Stamina though, compared to the clay courts, not so much. Fast play is still fast play, however core taxing.
The Clay Court, French Open's Roland Garros surface.
Play -- slow, methodical, and defense based.
Bounce -- medium high to high, and typically slowest of the three surfaces.
Favored hit style -- full western grip, for more topspin. See: Clay court specialists / baseliners
Slow play, that's what makes this court so grueling in comparison to grass or hard courts. Where skill, core strength, strong serves, good volleys, sharp ball spinning skills, etc can all make the other 3 tournaments play fast and efficient, adding more friction from the clay surface makes any game 'hack' substantially less effective.
Nope. On clay surfaces, you must grind it out. And the fact that Roland Garros does 5-set matches, with no tie-breakers, sets up higher chances of 3-5 hour matches, especially into the finals.
Not only does a player still need to excel in all the same areas of expertise it takes to win anywhere else, they also have to employ insane amounts of stamina to play on a clay surface. Matches on average don't just last longer to determine a winner, but it also typically requires many more swings per match as well. Slower ball movement means it's that much harder to "one and done" or "ace" your opponent, as least not often.
Mentally and psychically, I argue you are most tested by a clay surface than any other surface in all of tennis. This in turn makes Roland Garros the toughest Grand Slam of them all.