Roger Federer is convinced: ". every game has become almost mental and physical .". Good physical and mental skills are therefore very important to be able to play tennis at the highest level. However, until now there was little information about how young tennis players perform and develop on physical and mental skills. From 2005-2018 we measured 833 young tennis players (10-15 year old boys and girls, top 200 youth rankings) in which several physical tests and one mental questionnaire were taken. We have been able to follow nearly half of this group for several years. In the area of physical skills (speed, strength and agility), we saw that the fastest young tennis players showed better performance (higher ranking). The effect of good physical skills on tennis performance was greater in girls than in boys. However, physical skills measured at the age of 12 could not predict tennis performance at age 14. After puberty, the differences in physical skills were no longer visible. The tennis players who showed a better tennis performance at a younger age continued to perform better on the court even after puberty. Perhaps there are other factors that play a role in tennis performance at this age. One of these factors may be the mental skills of the young tennis players. Tennis performance can partly be explained by mental skills. In conclusion, physical skills appear to form the basis for professional-level development, while mental skills can help players optimize their tennis performance.