When we want to analyse an athlete, it´s important we take into account 4 different components, as we´ve stated in a number of previous articles:
The player´s performance in a competitive context will be dependent of the interaction between all these components, because, at the end of the day, they all matter for a player´s overall performance level. However, the role of each component varies from sport to sport, which means that :
There are sports in which the technical-tactical side matters more than the physical one
There are sports in which the importance of the physical component is far bigger than the technical-tactical ones
In every sport there is a strong mental component, because in order to be an elite athlete there a few psychological traits you should have and that will help you have a more steady and organic growth throughout your playing years, such as: emotional intelligence, spirit of sacrifice, resilience, consistency and learning ability, for example
Regarding the physical component, we can define athletic development as being the growth in all physical qualities, to improve not only our player´s overall physical conditioning level but also to improve the global performance level, because there is a direct correlation between higher levels of fitness conditioning and higher levels of performance, according to the best scientific evidence available.
Today, more and more coaches begin to be aware of the importance of athletic development as a tool to improve they players playing level. However, we mustn´t forget that there are no medals to the player that lifts more at the gym. What does this mean? It means that no matter how much the players invest in their physical condition and how high their fitness levels are, that doesn´t simple guarantee that better results will be achieved. That is way the role of the athletic development may seem a contradition. No, physical preparation won´t win games by itself, but its crucial because no player can reach elite levels of performance without having good fitness conditioning levels.
That being said, we must understand the real importance of the physical component in tennis and how it actually impacts a player´s performance on court:
Athletes with better fitness levels will present themselves with better power, strength, speed, coordination and agility indexes, which will enable them to have a higher degree of proficiency in the technical movements they will have to execute while on court
Athletes with better physical preparation will have higher resistance to fatigue, which will enable them to increase the period where they can perform at the highest level without feeling tired. 
In addition, this also means that the players will maintain their decision making ability intact for longer, even in the latter stages of the game. Its common for us to see players who are technically and tactically gifted, but because of poor conditioning they reach a point during the game in which the fatigue levels will be so high that their judgement will be affected and their performance levels will start to go down. Higher conditioning levels can´t take fatigue out of the equation, because at some point of the game all players will feel some sort of exhaustion, but it can delay its appearance. 
Furthermore, better physical preparation will also increase the probability of having a consistent but solid performance on court, even on longer seasons.
The risk of injury in players with high fitness levels in considerably lower than the ones with less physical conditioning.
Players with less injury risk tend to have less injuries, with means they will have more playing time, and therefore more chances to accelerate their development..
 – Buchheit M., Laursen P. (2018). “Science and application of high-intensity interval training: solutions to the programming puzzle. 1st Edition. Human Kinetic Publishers.
 – Gregory Haff G., Travis Triplett N. (2015). “Essentials of Strength Training and Conditioning”. 4th Edition. Human Kinetic Publishers.
 - Gabbett T. et al. (2018). “Can the workload-injury relationship be moderated by improved strength, speed and repeated-sprint qualities?”