Her problem is there is no room to breathe against the best players. Her serve has improved but she only goes for an ace once every 3-4 games. She needs 1-2 free points minimum in every service game. Second serve is an automatic loss of point. She actually looks pretty good against most players, often hitting a flat skidding approach shot and comfortably disposing of volleys but when the balls come crashing through the court towards her, she has no choice but to lob them back.
Justine had great technique on her shots and just forced herself to jump into her shots, which gave her shots additional zip. Rad has great hands and deflects pace but it is just not enough. She needs to deflect and yet attack like Azarenka.
IMO her biggest roadblock is Azarenka. This is a player she used to compete well against but has now dominated her, thrashing her left and right. Azarenka does not possess the great pace of Serena or Sharpie and has a weak serve but she stands on the baseline and hits left and right with great depth. Taking balls that early and hitting them firmly robs opponents of time the same way Serena or Sharpie do it by hitting hard. In fact, if Azarenka could add some more power to her own game and serve, she would be unstoppable because she is just extremely solid (of course, if she hit harder, she might become less so).