Throughout the history of the game, there have been many U.S. champions who have won on both the amateur and professional stages.
For a long time, starting in the 1920s and extending through the 1960s, a player had to choose to be either amateur or professional. Without turning pro, the player could not win prize money.
This fissure divided the men's tour, ensuring that the best players did not face each other on the greatest tennis stages—mainly the four annual Grand Slam tournaments which remained strictly amateur.
That ended in 1968, when the "Open Era" in U.S. tennis swept the old system out. But it made comparing players across eras difficult because essentially some were playing in a system most do not comprehend today.
With the formation of the ATP, all players met on the same stage able to compete in all events, soon winning adequate prize money.
Throughout the years, before and after the Open Era, some U.S. champions changed the game with innovative approaches as well as humanitarian efforts off court. Some simply played the game more successfully than others of his era.
These factors entered into the this ranking of the top U.S. men of all time—including the recently retired Andy Roddick.