Our dog loves tennis balls, too, but I don't let her have them unsupervised since the dog of a friend of ours ate a tennis ball last year and needed expensive surgery to remove it from her intestines. It was not pretty. I also don't like the abrasion on her teeth.
But to let her have her fix, I put the ball in other toys. I have a stuffed kitty that I bought for $5 that has a pocket in the middle that seals with velcro. I stuff the ball in there and it takes her a long time to get it out. Or I wrap the ball in an old towel and tie it up with tug toys so she has to work to get at it.
We pick up the balls and put them away whenever we're through with a play session. Has the added bonus of keeping her interested in them, too.