Flow Blue de Matteru

The only worry for Ikuma Nagori, an elementary school student who's about perfect at everything he does, is that he has no rivals to compete with. What changed this was his encounter with baseball and Yurimoto Genji, a transfer student from Ehime. In order to "meet at the Koshien finals in eight years", the two genius boys put everything they have onto the field.

(Source: Kodansha, translated)

Note: Includes two extra chapters.