The 20-year-old Thai sank three birdies in each nine in his blemish-free round to be one shot ahead of a quartet that included compatriot Gunn Charoenkul and American Matt Kuchar.
"To be honest, I'm just really happy to be playing in this prestigious event," Hamamoto said.
"I just went out there to have fun and tried not to put too much pressure on myself.
"I've been trying to work as hard as possible after graduating from Qualifying School and the results are finally beginning to show."
Jazz Janewattananond of Thailand kicked off his title defence with a promising 67.
"The score is not bad although I left a bit out there," said the 2019 Asian Tour Order of Merit champion.
"It's still the first round, so there's no need to go too serious about this now. It's still a good score so I’m pretty happy with it."
Former world number one Justin Rose was further one shot behind though the Englishman was not too unhappy with his round either.
"My game was not exactly great. I did not hit many good shots today but I think I managed my game very well," Rose said.
"I could have played the par fives better... Those are the only parts of my round that I think I could have done better." (Reporting by Amlan Chakraborty in New Delhi; editing by Christian Radnedge)