Sadaharu Inui (乾 貞治, Inui Sadaharu?) is a highly intelligent player, who uses "Data Tennis." He constantly collects and analyzes data while in and outside of the tennis court in order to determine statistics, weaknesses, and other factors. While he collects data pertaining to tennis, he also collects personal pieces of data, such as favorite things, personal habits, and where they have been, which makes others feel awkward. His style originates from his close friend, Renji of Rikkai, who plays doubles with Inui before moving. He is temporarily removed from the regulars after losing to Ryoma and Kaido, and becomes the team manager. He uses his data to create exercise programs for the team, and creates a series of disgusting "Inui Juice" health drinks that are able to knock a person out, which motivates the team to train. He is voiced by Kenjirō Tsuda in Japanese media, voiced by Jack Bauer in English media, and portrayed by Hirofumi Araki in The Prince of Tennis live-action film.
Inui's data tennis allows him to predict the exact probability of a shot taking place, though other factors can disrupt the probability. Though he relies on data, he still trains excessively to keep up with his teammates, taking on a training load 2.25 times more than Kaido's, which is already far ahead of the rest of the team. He pairs with Kaido in doubles to help Kaido train, and Kaido's massive amount of stamina is used to let him play alone while Inui rests and gathers data. The style does not work against Ryoma, who constantly evolves throughout each match, and he finds it difficult to collect data from players who do not play at their full potential, such as Tezuka and Fuji. He develops two extremely fast serves that are among the fastest in the Japanese tournaments. The "Sonic Serve" is developed first, which is trained with wrist weights, and the "Waterfall" serve, its arc and descent resembling that of a waterfall engulfing the opponent, is developed afterward.