I can't say that I'm totally surprised given that she's always been inconsistent, but this summer there seems to be something else at play. It's been a crazy year for her with the AO final and FO win, and becoming a mega celebrity in China. She seems a little bit stressed, a little worn out. Now the Slams are over. Time for her to rest.
And, as much as she improved her game, it is still a game that can have wild fluctuations. Even though she's learned to play more recovery shots and rally balls, she still is an aggressive, hard hitting player, and once those shots stop dropping in, confidence can decline quickly, Slam champion or not.
As I said in another thread, I don't know if she'll win another Slam. She certainly could if she gets back in form, but I'm not going to count on it. Her late career run of AO SF, other Slam QFs, AO final, and FO win, is pretty remarkable. Regardless of whether she wins another Slam, I do think we'll see her have good Slam results (QF or better) again. It's not that uncommon for players to have a rough second half of the year. I don't think we've seen the last of Li Na playing at a top level.