When Del Potro was still new to the tour, Federer noted that he (not an exact quote) "played strangely - he's a big guy but he plays like a small guy".
When he stepped up his game late in 2009 he finally started to play like a big guy. After his return, he's played more like he used to pre 2009. This was the first match of him that I saw where he played like a big guy again. Good signs for him, bad for the rest.