Several skill tests have been developed to assess the level of proficiency in forehand and backhand groundstroke techniques several decades ago, including the International Tennis Number (ITN) 10.3 Test by the ITF. However, the validity, reliability, and suitability of these tests may not necessarily be guaranteed when used to measure proficiency in these two techniques. This research aims to examine the level of validity, reliability, and relevance of the ITN 10.3 Test in relation to the proficiency level of forehand and backhand groundstroke techniques among junior tennis players aged 12 and below in the Special Region of Yogyakarta Province. This study is a quantitative descriptive research using a survey method. Data collection techniques involve tests and measurements. The population in this study consists of junior tennis players in the Special Region of Yogyakarta. The research sample was selected using purposive sampling technique, consisting of 30 male and female junior tennis players aged 12 and below in Yogyakarta. The research instrument uses the "ITN 10.3 Test" to measure the basic technical proficiency of forehand and backhand groundstroke. Validity test data analysis is performed by correlating the best results from the ITN 10.3 Test with the results of half-competition matches using Pearson Product Moment Correlation, reliability test using Pearson Product Moment Correlation analysis technique, and relevance test is analyzed using T-Score scores. The research results show that the validity of the ITN 10.3 Test is -0.976. The test reliability is 0.982, and the test relevance is categorized as moderate, good, and very good at 69.99%. Thus, it can be concluded that the ITN 10.3 ITF Test can be used as a standardized assessment instrument to measure the proficiency level of forehand and backhand groundstroke techniques among junior tennis players aged 12 and below in DIY.