The earlier you start the better. And if you want a better answer, I would say high school sophomore year.
You need to really sit down and and research the schools and what are your goals not only for tennis, but for the academics as well. Once you have your school picks, make sure you learn enough about it. This will really help you ease your conversations with the coach when you finally contact him/her.
Building a relationship with the coach and the team is really important. Just so you know you are allowed to call or email the coach at any point in time. So take advantage of this opportunity, and show your interest. It is always nice to have some help during your search, but this is really about you. Coaches are not only looking at your tennis skills and records, sometimes it comes down to personality.
Take it serious, just as if you were getting ready for a job interview.
Start networking early!
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