No doubt for more than just fatigue. There's a reason most of the longer matches occur between lower-ranked players; neither opponent has the mental fortitude to change strategy accordingly. Not to be mean, I do concede that Isner and Mahut both have heart to fight for 7 plus hours, but this wouldn't happen between Federer and Nadal because they learn and adapt during a match, not just go tit for tat. Both of these guys serve very well, but return very averagely.