Tennis has always been called an easy sport, usually by people who do not participate in the game. This may be because there is no game clock, and players are given brief intervals after every point, making tennis seem a slow game or a relaxed game. However, this is not true, and the game is not easy at all. One of the critical factors that make this sport so challenging, and which most players are well aware of, is how it physically challenges a player’s energy and physical condition. 
According to a study by Human Kinetics, after about an hour of playing tennis, striking the ball inaccurately, unforced errors and mental mistakes begin to happen, serve and stroke velocity decline and your speed drops. This proves how physically demanding the sport is.
Playing tennis can take several hours, and a player might need to sprint and stop on the court repeatedly, which is hard to do, especially since, unlike most team games, the players are never substituted out of the game.
According to a study by GoTennis, the game of tennis requires a very strong mental game, too. Dealing with failure is very tricky, and most tennis players give up at a basic level. All of the pressure is on you compared to a team sport, where it is a team effort, not based off of one player’s success. 
According to another study by Sports Psychology Coach, most players would say tennis is 90% mental and 10% physical.  It is relatively easy to hit a tennis ball but overcoming mistakes and not getting down on yourself can seem impossible. During tennis, the players are left to themselves, and nobody supports them. If any player makes a mistake, then the whole game may change. A player must remain calm, patient, and relaxed, particularly during intense pressure situations. In the game of tennis, mental toughness plays an important role in success.
Another problem faced when playing tennis is the high skill levels required to play the sport. For example, a serve, a forehand, or a backhand takes a long time to master. This means that even the smallest mistake can cause the player to miss a shot or a point. This is unlike some other sports, where hard work alone can guarantee success in a game.
In conclusion, the reality is that the game of tennis is not a simple game, which can be ascertained from the fact that the game requires a person to have physical and mental endurance, skill, as well as consistency. It is therefore quite obvious that it is not easy to be good at the game of tennis and should be considered one of the hardest sports.