EA SPORTS MMA

During the course of career mode you probably be given a rematch (I say probably because you don't pick your opponents and I've done entire careers never getting a rematch.) In order to get a rivalry fight you have to LOSE any non-title rematch. The next fight should be a rivalry fight against the fighter you just lost to.
NOTE: This must be done before attaining a title. Losing a rematch for a title will NOT get you a rivalry fight. You have three leagues to do this in, so if you still dont have it after you win the Strikeforce or Mystic belt than you can just lose a couple fights to rank down then start winning again and hope you get a rematch. The career mode is always 40 fights long.

I think there may need to be a slight adjustment to the solution. I don't think losing a rematch will always give you the potential for a rivalry fight. Rather, the score has to be tied after a rematch. In other words, if you beat someone and they challenge you to a rematch, then you have to lose. However, if you lost at first, then you should win the rematch to have a shot at the rivalry fight. The phone messages between each fight kind of hint at this.
Also, jsigversion1 has posted something similar at X360a with this--"ok, i figured it out... when you get a rematch fight, do the opposite of what you did [at first] so you are tied after it at 1 win a piece, down the line you'll get him again [for the rivalry], win that one."

One other thing--after I tied things up with three different fighters via rematch in the last leg of my career, I still couldn't land a rivalry. I was running out of chances before the career ended. So, I started dashboarding before I accepted the contract on one of my last fights. I didn't want it to save and lock in the wrong fighter. Every time I reloaded my career, it would give me a different guy to fight next. Other words, it's pretty random. After about the fourth dasboard, it finally gave me a rivalry.

Also, rivalries don't need to be directly after the rematch. They can pretty much spring up any time within the league you're a part of. To reiterate, they just need to be set up properly as tied after the rematch. If you do this, you'll probably just be given a rivalry if you stay in the league you're in. If you start nearing the end of the career, try the dashboard trick. Worked for me.

Also, important note: As has been said here before, it can't be contender or championship fights. They won't unlock the achievement.

Hope this helps.

EDIT: So it looks like contender fights can still count as part of this process. See comments. Championship do not, though.

What I did was:

Win the amateur fight
Then on your first professional fight (I fought somebody Christmas), lose your debut fight
Then WIN every single fight in your career, by the 5th or 6th fight you should be facing your debut opponent for the 3rd time.

I did this offline, so I am unsure if it works online as quickly, but I got it so the above method works



... win contender match then lose big match then you should face the rematch then win . contender fight and win .. then championship belt lose ... after that you will get a rivaly match