How do you say 'A little rusty' meaning out of practice in Chinese?

A little rusty:
If you used to be good at something, but because of lacking practice, now, you don’t know how to do it properly.
You could say “有點生疏”. Pinyin (you dian sheng shu), which means “not very familiar with it but still not bad”.
Ex. I’m a little rusty on playing tennis.
我對打網球有點生疏/不熟(悉)了.
Pinyin: Wo dui da wang qiu you dian sheng shu/bu shou (xi) le.　

If you totally forget about something you used to have the ability to do it.
You could say “我忘光了”Pinyin (wo wang guang le), which means “I forget it all”.
Ex. I forget calculus all.
我對微積分忘光了.
Pinyin: wo dui wei ji fen wang guang le.