That competitive third set was partially due to Sela raising his level. We see it all the time where a player plays so poorly for a while that when he raises his level slightly, his opponent isn't quite prepared initially because he hadn't had to do anything special for 2 sets. That's definitely part of it. The other part is Sela started slightly figuring out what he needs to do against Nadal after initially being completely taken aback by Nadal's spin. That's the sort of thing you can't be prepared for unless you play Nadal relatively frequently.
Nadal on the other hand still shows his lack of match-play recently as he's struggling to maintain the same physical and mental intensity throughout, but that was definitely a far better performance than the one he displayed in the second round, although the nature of the match-up facilitated that. The Anderson match will be pretty telling.