I think it's mostly genetic, childhood education or dramatic life changes. I have rarely seen anyone who has zero discipline slowly acquires very good discipline without any significant changes in his/her life.
I made the mistake of reading a number of parenting books, and they pretty much all start out with some form of: "You must be self disciplined or this method won't work."
In one case it was just sheer ambition but he wasn’t poor to begin with and in the other it was practicing a religion focusses on positive thinking which she adopted that also buikt her determination.
It is interesting to juxtapose with Agassi and The William sisters.
These two people are also on the top 0.1% Of their respective disciplines.
About William sisters, wasn't their father the first coach? Not to indicate anything but it falls in childhood experience catalog I think.