Australian Day is keen to impress International captain Ernie Els for a spot at December's Presidents Cup in Melbourne and the 31-year-old wielded a favourite putter to sink six birdies at Nine Bridges Golf Club on Jeju island in South Korea.
"I think overall it's a great start...to be able to capitalise on a day like we had today because we're not used to the wind being down like it was. I'm excited to see how the rest of the week goes," Day said.
An also had occasional trouble with the swirling wind and the Korean was happy not to drop a single shot.
"Eight under, bogey free is always good," he said after carding a sparkling 64 to finish one shot ahead of Joaquin Niemann.
"I didn't hit it great the last couple days and on the range today, but everything seems to be working," added An.
"Everything was going at the flag the perfect distance and obviously I holed a lot of putts today, so everything just clicked and I shot bogey free."
Brooks Koepka launched his title defence with a modest three-under 69 with an eagle on the final hole in the first tournament of PGA Tour's three-event Asian swing.
Inaugural winner Justin Thomas shot 68, while fellow Americans Phil Mickelson and Jordan Spieth returned 70s in their tournament debut.
Rookie Viktor Hovland shot 69, his 18th straight round in the 60s, which is the most by any player since 1983.
(Reporting by Amlan Chakraborty in New Delhi; editing by Toby Davis)