Hajime no Ippo: Fighting Spirit!

From K Manga:
What does it feel like to be strong? That's the question on the mind of Ippo Makunouchi, a clumsy high-school kid who gets bullied a lot. But when he runs into pro boxer Mamoru Takamura, it isn't long before the dynamite in his fists is unleashed! Dreaming of joining Takamura in the pro ranks, Ippo joins him at the Kamogawa Boxing Gym and there his quest for true strength begins, in and out of the ring!

The oneshot Hajime no Ippo: Gaiden - The Naniwa Tiger was published as chapter 806.5 in Vol 84.