Complex sentence: The Grand Slam Cup was not recognized by the Association of Tennis Professionals (ATP), however, and thus participation in it did not count for any points in the world rankings and it was not considered an official career title.
Split into simpler sentences: The Grand Slam Cup, however, was not recognized by the Association of Tennis Professionals (ATP). Therefore, participants did not receive points in the world rankings maintained by the ATP, and the event was not considered an official career title.