Eons ago, I played basketball Friday night and Saturday night. My legs were always fatigued on Saturday a bit. I was in terrific shape, but I could feel the difference. I don't recall ever having a problem being "up for the game" though. We didn't play that many, 20 a year maybe, and each game was to be attacked in my mind.
Some of this can just be random variation, you have a great game one week and a tanker the next. People look for a reason. Some times it's just random variability.