Hi
@GlyphXion,
In sports, a "classic" is a term used for recurring significant events, as well as many of the most significant rivalry matches in sports (especially in soccer/association football).
For the latter, you probably came across El Clásico, which is one of the biggest rivalries in all of football between Real Madrid and Barcelona. There's also the Super Clásico between Chivas and Club América in Mexico, California Clásico between the San Jose Earthquakes and LA Galaxy in California, Der Klassiker between Bayern and Dortmund in Germany, and many more.
For baseball specifically, there's the World Baseball classic, which is essentially a baseball World Cup; and the World Series (playoff finals series in MLB) is also called the "Fall Classic." There are similar phrases used across other sports and seasons.
As such, translating that phrase as "The Classic" is entirely appropriate, and more understandable to a sports fan.
...Actually, I just checked the en Wikipedia, and Giants-Tigers is specifically listed as "The Classic Series", and I'm guessing the term used was 伝統の一戦.
https://en.wikipedia.org/wiki/List_of_sports_rivalries#Nippon_Professional_Baseball
Edit: Oop, before I forget, thank you so much for translating this series! I've been looking forward to reading it.