This year, in 2019 the ITF revisited the rules for the pro tournaments. To get ATP points (Professional point corresponding to your ranking), one player has to go through a lot of steps. First, he has to go through the pre-qualification of a tournament $25,000 minimum, then, if he wins those rounds he can go to the qualification draw of the same tournament, and after winning all his rounds the player can participate in the main draw of the tournament. He will earn one ATP point only if he is in the final of the main draw. Before that, the player won't earn any point.
But here is the problem: one point is nothing. For example, Dominic Thiem, who is ranked #5 ATP has 4765 points. The top #500, Giovanni Fonio has 11 ATP points. So the difference between top-ranked players and lower ranking is huge. To get to the same level, lower-ranked players need to do years of tournaments or have a great win one day.
More than being a highly competitive sport, tennis is also an extremely expensive sport. Let's take the example of the same $25,000 tournament. A player will have to pay $50 dollars to go to the pre-qualification and then another $40 to get in the qualification before being able to make it to the main draw. When in the main draw, a player really makes some money when he reaches the final and of course if he wins. But even in winning, the money earned probably doesn't cover all the expenses of the player to practice, pay his coach, and the expenses related to the tournament.
To conclude tennis is a highly competitive sport where it is a hardship to get a good ranking and is also an extremely expensive sport that can put you in debt if you don't have the victories.