I've noticed that at tournaments 2 things are very important:
By the semis or finals, you've played 4+ matches in 2 days. Those are not normal conditions, and they favor the athletes in the draw.
And you have to keep the ball in play because most of the players get nervous in tournaments...these situations favor the more consistent (if not flashy) players.
Thus, at the end of the tournament the winner may not necessarily be the "best" player...the one who on 3 days of rest could beat pretty much everyone in the draw.
Same reason the grand slams are so difficult to win in pro tennis - you have to win 7 best-of-5 matches in a row! It takes place over 2 weeks instead of our weekend USTA tournaments, but takes a toll on even the most fit players. It is a sport after all; fitness and consistency ought to count for something.