Comparisons to other returnees this year in their first two tournaments;
Raonic; Lost first two matches. Once to a teenage rookie, one to a journeyman.
Wawrinka; Got Whooped by Tennys, winning only 7 games in 3 sets. Then lost to a journeyman.
Nishikori; Lost his first match in a challenger. Then won another challenger but has no notable wins on the ATP this year.
Nadal; Got injured again
Djokovic; Lost to "baby Djokovic" and to a journeyman. Real question marks over his fitness and attitude.
Tomic; Lost in qualifying for the Australian open then to the world number 253 in a challenger. Sorry couldn't resist throwing him in
I wouldn't say Serenas comeback so far has been out the norm, considering all the complications with her pregnancy.
Fedster comeback was the exception not the norm, and their situation was not as serious. The best comparison to Serena i can think of is James Blake, in terms of the seriousness of the condition that kept him out. He spent months on the challenger circuit to recover his game.