"The mindset of the Japanese players has changed," Leitch said.
"We are playing with a lot more confidence. Today's performance showed what we can do and we are only going to get better. We had our chances but couldn't execute."
The Brave Blossoms have not managed to beat a tier one side in seven attempts since their stunning victory over the Springboks at the last World Cup.
The result left Japan with one win, one loss and one draw in the test window after a heavy defeat to Australia in Yokohama and a victory over Tonga in France.
"I am disappointed for the team, we deserved to win," said Japan coach Jamie Joseph.
"We haven't been that successful against tier one sides since 2015, so this is a step in the right direction."
(Reporting by Aditi Prakash in Bengaluru)
