Del Potro had to beat Federer, and had one day less to rest than Nadal. I think a two day full rest should be available when we go for semi-finals and finals. I guess this wasn't really fair for Del Potro but it's how the tournament is run.
I think its less than 48 hours before he faces the Nadal so I guess he's body wasn't able to recover from the Federer match. But I do agree that its not fair specially that players have to go a lot of matches before reaching the semis and its getting tougher and tougher as you go along. If we all know that he will be beaten by Nadal, we just wish that Federer should have won then to see Federer-Nadal once again. Oh well, its gonna be easy for Nadal to take the trophy against Kevin Anderson. I'm not saying that Kevin can't win, but knowing Nadal in this kind of tournament, its really hard to go against him and pick Anderson.