Matt Seeberger is probably the most successful division III player of all time. His record was 102-6 overall. He won 3 National DIII singles titles and 3 doubles. After seeing that, I figured he could potentially get an ATP rank in the top 1000 or so, but then I saw that he has been playing open events and although he has won some, there are guys that have beaten him. This seems to kind of disprove the "Divisions don't matter in tennis" theory everyone postulates. If someone had that kind of success in D1 they would definitely be legit pros.