Yes, Wang Yihan was simply unrecognisable in that match with Marin. As reported here
http://sports.qq.com/a/20140829/007193.htm, she explained (excuse or not, up to you to interpret) that partly she was beset by injuries which disrupted and affected her during the training period before the WC.
But, largely, she blamed herself for the dismal performance, saying she was unable to get her game going, just couldn't string her shots together, and committed too many errors when attacking; furthermore, her movement was sluggish which hampered her overall game. That, in addition to her opponent getting buoyed up and playing with greater intensity to secure an unassailable lead, finally broke her down.
Personally, my concern is that wasn't the first time she suffered such a collapse, a sort of brain fade (i.e. lapse of concentration leading to disaster, a term which comes from auto racing). Remember those recent one to years unexpected defeats to Nichaon Jindapon, Sayaka Takahashi, Yao Xue, Shizuka Uchida, Yip Pui Yin, Lindaweni Fanetri, etc, some of them early exits. In fact, her form after the LOG'12 has taken a significant dip from which she never recovered. Perhaps her losing the Olympic gold to Li Xuerui has something to do with it.
Nevertheless, as somebody encouragingly said, WYH admires her mentor Zhang Ning who was a late bloomer, playing her best badminton in her later years, after age 26 or so, and winning two Olympic golds, the second of which at age 32. So let's hope she can take inspiration from Zhang Ning, emulate her and continue to strive on to greater heights going forward. Wish her every success. JiaYou !