As a tennis player, I think I am insulted
Seriously, I can tell you from having visited a tennis blog exactly once, the hatred of Habs fans for Leafs fans in no greater than Nadal acolytes for Federer fanatics.
I don't know why, maybe I didn't hate the Finn/Swede battles so much because of I was friends with my tennis friends, broke barriers when it wasn't cool to be a nice person kinda (because of our history). And Swedish is still our second language, rare some to speak it or be good at it except those swedefinns. When the Finns lost and Swedish won against us some earlier years (in hockey champs) when I still had some promise. After the loss which we watched together, I asked my friend will he coming to tennis practise tomorrow. He didn't say a word. Not a word. I mean nothing. So I eventually left. And as I started tennis as a age of 6-7 years old, I've been the neutral thing in tennis.
It's just so weird to hate that much, though I've watched forums of tennis that like you mentioned... it's life or death for a fan with Nadal and Fed. I can see the sides, but.. man someone got issues for sakes of a sport.
Can't tell if it's empathy or what in me that broke the hatered barriers in the first place (now it doesn't matter, I know that here people are friends and open about it no matter what is the language), but still... it's a weird concept to hate the other team so much. I mean support your own, but so deep hate. It's weird for me.
TL;DR: (again) I know the concept of hating rivalries and someone in the way sort of speak, but I've never hated even the people that have beat me in some tennis finals, many many times. I'd take it's some cultural and age thing since my father (rip) hated swedes in Finland, when I was always trying to change his mind. (Why do I always write a novel for the simple things? I guess we never know, but maybe after Aho 6 point game in the playoff opener will tell the frogs for me what's in motion, can't be stopped).