i suppose in an ideal relationship, the friend would call a somewhat equal amount of time and ivite you to do things or atleast call during the week and say "hey we are still playing this weekend right" and seem interested.
Why?
What if you are the organized one and your friend appreciates that you are the stable one that takes the time to plan and organize your weekly tennis game? Maybe your friend is disorganized and/or flighty? What if she has things going on in her life like children, busy job, etc. and while she might not find the time to call and reassure you that she loves playing she really DOES look forward to playing tennis with you? That's why she says yes when you invite her.
What if she is slightly intimidated by you and if you don't call she feels like she would be a burden to call and bother you? Or maybe she thinks your life is so busy and doesn't want to bother you if it appears your life has gotten hectic?
To me if you really want to play tennis it should be enough that you want to play tennis, you ask your friend to play and she says yes and shows up to play.
But what is happiness except the simple harmony between a man and the life he leads? ~Albert Camus