TURIN, Italy — While Olympic champion Kim Yu-na crumbled, American Mirai Nagasu soared.

Nagasu was in first place after a nearly flawless short program at the World Figure Skating Championships on Friday. Kim, who has lost only one competition over the last two seasons and was downright majestic in winning gold in Vancouver, was in seventh place after three major errors in an uncharacteristically sloppy performance.

Coming off the ice, Kim told reporters that her left foot was "shaking." But it was unclear if there was a medical issue, and coach Brian Orser couldn't be reached for comment.

"The first triple combination was perfect, then I felt not sure on my left foot. It was shaking. It wasn't feeling good and I don't know why," the South Korean star said.

Nagasu scored a season-best 70.40, putting her two points ahead of Olympic silver medalist Mao Asada of Japan — and more than 10 in front of Kim. Finland's Laura Lepisto was third. The free skate is Saturday.

"I think they will be back on their game tomorrow," the 16-year-old Nagasu said after seeing Kim and Asada skate. "I am sure they are tired after the Olympics, as I am, but I just have to concentrate on myself and do the best long program I can."

Later Friday, Olympic ice dance champions Tessa Virtue and Scott Moir try to wrap up their first world title. The Canadians have a slight lead over Olympic silver medalists Meryl Davis and Charlie White going into the free dance.

Kim has been in a class of her own the last two seasons, and she capped it with her dazzling performance in Vancouver. Adored in her native South Korea, she handled the massive expectations with cool grace, setting world records for both her short and long programs. Her total score was more than 23 points better than Asada's, a massive rout.

But there's a reason many of the Olympic gold medalists skip the world championships, held just a month after the games end. There is bound to be a letdown after achieving the sport's greatest prize, and it's hard to get back into training.

"One week ago, before I got here, I was a little bit scared," Kim acknowledged. "But I was ready."

She didn't look like herself Friday.

She received no credit at all for a spin, underrotated a triple flip and bungled a spiral that received only a first level and was downgraded on execution to just 1.46 points. Not even her strong opening triple-triple combo could make up for that.

Her score of 60.30 points was more than 18 points behind her record-setting performance at the Vancouver Games.

"I felt very good at the warmup. My first jump, the triple-triple, was really great. And then after that I felt I was ready to do a triple flip. It was really weird. I don't know what happened," Kim said. "It's the first time I missed the elements like that."

Nagasu won the U.S. title in 2008, but then struggled with a growth spurt and ordinary teenage angst. She switched to coach Frank Carroll last spring — she now trains alongside Olympic men's champion Evan Lysacek — and the move has done wonders for her skill and confidence.

She was fourth at the Vancouver Games, and has made it clear she wants to be the one to watch in the lead-up to the 2014 Sochi Games. This was a good start.

Before skating, she spent a few minutes at the board talking to Carroll, holding both of his hands and concentrating on his words.

"She said she was scared. I said, 'Well, you said to me you wanted to be the Olympic champion someday,'" Carroll said. "I said, 'Future Olympic champions don't get scared. They get tough.'"

Though her opening triple-triple combination was slightly underrotated, the rest of her short program was exemplary. Lysacek, not competing in Turin, was ecstatic with his training partner's success, posting a message on Twitter that said, "Wow!!!! So happy for you!"

Nagasu has been working to get the triple-triple into her short, and credits Carroll for getting her there.

"I just listened to my coach," she said. "He just has a positive aura about him."

Carroll said he just reminded her that she wanted to put the triple-triple in at the Olympics and didn't.

"So what are we here for? ... Back up your words with action," Carroll said.

Asada, the 2008 world champion, trails Nagasu largely because she underrotated the triple axel in her opening combination.

"Of course, I am very disappointed about the downgrading of the axel. However, I think I was able to perform relatively well. And I think I picked up the right momentum going into tomorrow's long program," she said.