Shamo and Zero portray combat sports as a way of living and not as a sport, something to hold onto when nothing left. Both they address issues like fighting sports as a business, managers and the fighter as merchandise.
The MC’s are not concerned about the success and reputation, even for being particularly virtuous in their discipline, their thoughts and feelings are outside of the entourage and system in which they are involved. Their only concern is to demonstrate its superiority to that or those that pose a challenge, cause that’s the only way for them to feel alive.
Both are sports manga by Taiyou Matsumoto, and as such, they have many similarities. First off, both manga have a similar art style, which instead of words, relies mostly on imagery and metaphors. They also have an unorthodox approach and are unique in the way they show sport matches, opting to use sport as a means for the main characters to gain a better understanding of themselves and their lives.
Both are sports manga that use boxing as a tool to show the main character's growth and development. Both focus on a strong main character, who is constantly looking for strong opponents to fight against. They create an overall similar mood.