Originally Posted by Mewtarthio
It's not so much "The last fight is the toughest" that's the trope. It's "The last fight is the most climactic." As in, if you have any major rival in the tournament, you will fight them in the last round. If there's someone you hate even more (say, the Philadelphia Cacti are your friendly rivals, but the Cincinatti Octopodes burned your hometown and killed your coach), you will fight the rival on the second-to-last round and the villain on the last one. That's unrealistic. Particularly in sports, since rivalries usually form between teams that can't face each other in the finals (eg, in US football, rivalries tend to form between teams in the same division, while the Super Bowl is fought between teams from separate conferences).
Actually, in some sports you can face your rival in the finals. For example, Roger Federer and Rafael Nadal in many tennis grand slam tournaments*. In addition, some tournaments don't have the early rounds by divisions or conferences. Granted, you can't have a Red Sox/Yankees or Cardinals/Cubs World Series or a Giants/Cowboys Super Bowl. But you can have a Manchester United/Manchester City FA Cup Final or a North Carolina/Duke NCAA Basketball tournament final.
*This is probably the closest real life analogy. Federer and Nadal have faced each other ten times in grand slam tournaments, eight of those ten were in the finals.