Slow Step(1986)

Slow Step

High school romance, softball and boxing story. This manga focuses on the love triangle between Nakasato Minatsu, Akiba Shuu and Kadomatsu Naoto. Due to a strange set of circumstances, Minatsu (a softball player) is disguised as another girl when she bumps into Naoto (a boxer), who falls in love with this "version" of Minatsu.

(Source: Prisms)