HAMILTON, Ontario -- Tanith Belbin and Benjamin Agosto had a blast, and Marie-France Dubreuil and Patrice Lauzon had a breakdown.
The U.S. ice dancing champions jitterbugged into first place at the Four Continents meet by getting top marks for their original dance Thursday.
"This program is so much fun for us and it gets better and better every time we do it," Agosto said. "It's a blast to do."
Canada's champions had an uncharacteristic fall in relinquishing the lead. Lauzon stumbled to the ice and Dubreuil went down with him 10 seconds before the end of their 2 1/2 -minute routine.
"I don't know what happened," an embarrassed Lauzon said. "I felt my (blade) edge wasn't right - I was on the wrong edge.
"We've never fallen in the original dance, not even in practice."
Belbin and Agosto saw them fall while glancing at a monitor during their warmup under the Copps Coliseum seats.
"That's such a shame, because it was such a good performance," said Agosto.
The free dances will be performed Friday.
"We will move forward," Dubreuil said.
Melissa Gregory and Denis Petukhov, No. 2 in the United States, jumped into third place from fourth.
Belbin, 19, who was born in Kingston, Ontario, trains at the Detroit Skating Club with Agosto, 22. They got the first 5.9 of their careers. It might be their last, too, if the interim judging system being used this week is phased out before next season by a new points format.
Americans Amber Corwin and Angela Nikodinov are 1-2 heading into the free skate.
Corwin, fourth at the U.S. championships earlier this month, edged Nikodinov in the short program Wednesday thanks greatly to a triple toe loop-triple toe loop combination as part of a solid overall performance.
"It felt strong," she said of the combination. "I feel like it's my trademark, my secret weapon.
"This is my world championship. I felt fabulous out there. I felt like I was flying. It was magical."
The free skate is Friday night.
Nikodinov already owns a complete set of Four Continents medals, but she has struggled since her 2002-2003 season was cut short by shoulder surgery. She'd been to only three competitions this season and was fifth at nationals. She landed a triple lutz-double toe loop combo smoothly and made no major errors.
"The lack of competition I've had affected me more mentally," she said. "You learn from competing how to get into the zone, how to be in the moment. Everyone is great in practice without the pressure, but once you turn that pressure on, the lights go on and the show is on, it's a different story."
Yukina Ota of Japan was third, followed by Canada's Jennifer Robinson.
The meet is for skaters from the Americas, the Far East and Australia, but many top American, Chinese and Japanese skaters are not competing.
Later Thursday were the pairs and the men's short programs.