...
TennisId is the unique identifier for a player within ITF. UAID is the unique identifier within USTA. Client applications can choose to display tennisId and deep link into ITF’s public website internationaltennisnumber.com which shows additional information about a player. There are no credentials required to access the ITF website. It is open to all.
Example: https://worldtennisnumber.com/eng/player-profile?id=6127768d9a88a03bfb1ac01d