South Korea's Kim, who wowed Vancouver when taking gold with a record score in last month's Winter Games, blamed a foot problem after finishing only seventh with 60.30 points in the short program at the world championships.
The defending champion could struggle to make the podium after Saturday's free program with her big rival from Vancouver, Japan's Mao Asada, second behind American Mirai Nagasu. Finland's Laura Lepisto lies third.
"The first triple combination was perfect than I felt not sure on my left foot, it was shaking and I don't know why. My layback spin, my spiral and the footwork -- I don't know what happened," a stunned Kim, who trails Nagasu by 10.10 points, told reporters.
"But tomorrow I will fight because my motivation is still high and this will be the last program of the season. The Olympic Games were my goal and I wasn't sure if I could fight again for the world championships."
After leading the compulsory Golden Waltz on Tuesday and topping Thursday's original dance with a flashy Flamenco, the duo finished second in the free dance but claimed gold with a combined total of 224.43.
Their friends and training partners, Americans Meryl Davis and Charlie White, imaginatively used a musical montage from Phantom of the Opera to win the segment and take silver just like in Vancouver.
Italians Federica Faiella and Massimo Scali sealed bronze and were bombarded with bouquets and teddy bears from a flag-waving home crowd but they will now consider whether to retire.
(Writing by Mark Meadows, editing by Pritha Sarkar; To query or comment on this story email sportsfeedback@thomsonreuters.com)
