In what year did the ATP introduce the current ranking system? The answer is 1973.
The Association of Tennis Professionals published its first singles rankings on August 23, 1973. The system was created to make tournament entry and player selection more objective, replacing decisions largely controlled by national federations and tournament directors.
Ilie Năstase became the first player ranked world No. 1 under the new system. The original method was based on averaging players' results, and its details changed several times during the 1970s and 1980s. Weekly rankings became routine from 1979.
A common mix-up is 1990, when the ATP replaced the averaging method with a “best of” system modeled partly on competitive skiing. Another nearby date is 1976, when ATP doubles rankings were first published. Thus, 1973 is the canonical answer for the introduction of the ATP ranking system, even though the calculation method has evolved substantially since then.