Two beautifully-executed quadruple jumps made up for an error on the Lausanne 2020 champion's attempted triple Axel, which was popped.
The Lausanne 2020 Winter Youth Olympic Games champion, skating energetically to the high-tempo sounds of Vocussion by Yo-yo Ma, had the crowd clapping along as he nailed his quadruple Salchow-triple toeloop and quadruple toeloop jumps with high grade of execution scores – 3.10 and 3.61 respectively on the -5/+5 scale – from the judges.
Although the 17-year-old popped his planned triple Axel down to a single – worth no points – he still received the highest technical element (47.31) and program component (39.95) scores of all 10 skaters in Osaka.
It was a beautifully-choreographed routine, high in difficulty in both jumps and artistry.
The Four Continents bronze medallist, making his senior Grand Prix debut, received a total of 87.26 points.
"I was really, really frustrated with myself," Kagiyama told media afterwards about his mistake. "The first half of the programme was better than anything I had done in training – Salchow, toeloop included – and I thought this was going to be good. I started thinking too much, trying to perfect it and couldn't move the way I wanted to.
"I thought I was close to perfect on the quads and I'm pleased about that because they were better than anything I was doing in practice.
"The way I see it I'm starting all over again because this is my first senior competition."
Tomono Kazuki placed second on 83.27 points, while junior Honda Lucas Tsuyoshi – who skated first of the ten competitors – held on to finish third (79.22) in the short program. An error-strewn program from PyeongChang 2018 Olympian Tanaka Keiji left him fourth.
Along with Kagiyama, Tomono was the only other athlete to break the 80-point mark on Friday.
The 2018 Rostelecom Cup bronze medallist is searching for his second career Grand Prix podium finish, and aside from an error on his opening element put in a solid routine.
Tomono landed off-balance on the first jump of his quadruple toeloop-double toeloop combination and had to take an extra stride to adjust for the double.
However, he made up for that error with a clean quad Salchow and later a triple Axel.
"I expected more from myself in terms of points but I can build on this for tomorrow. I can do better," Tomono said.
"I hope to show everything I've been working on up until now.
"One of my goals for this season has been to be consistent, do well in both the short and the free. I'll take it one step at a time."
Honda, meanwhile, did not have a quadruple but strong elements – a triple Axel, triple flip-triple toe (the flip was called for an unclear edge), and a triple Lutz – gave him a solid technical score, which made the difference with Tanaka.
"I'm surprised to be where I am," Honda admitted.
"I was hoping to be in the second group tomorrow but I didn't think I'd place this high after the short."
Tanaka put his hand down on his opening quad Salchow, which was also marked down for a quarter under-rotation, and popped the flip of an attempted triple flip-triple toe combination down to a double.