Tennis pro tours and tournament ranking series

For many years before the Open Era of tennis in 1968, the usual format for the handful of touring tennis professionals was a series of two-man one-night stands across the United States and often in other countries as well. The most notable of these tours were the "World Series" or "World Professional Championships", in which the reigning world champion went head-to-head against a challenger, most often the leading amateur of the previous year who had just turned pro.

Source: Wikipedia — Tennis pro tours and tournament ranking series (CC BY-SA 4.0)