I ain't as good as I once was but I'm as good once as I ever was [closed]
I ain't as good as I once was...
I'm not as good now as I used to be...
...but I'm as good once as I ever was
...but for one time only I can be as good as the best I was in the past.
That is, in general, he's deteriorated, but with a great effort he can equal his previous standard.