Rai, who broke through for his first win last year in Singapore, was the best placed Indian at tied ninth, while Anirban Lahiri, a two-time winner on Asian tour, shot even par 71 to be tied 18th.
Two other Indians, Chiragh Kumar and Ajeetesh Sandhu, had a rough day. Chiragh shot a five-over 76 and was tied 81st while Sandhu struggled even more during his round of 78 to be tied 109th.
Rai was three behind co-leaders, the 56-year-old Boonchu Ruangkit and young 17-year-old Miguel Tabuena of the Philippines, who shot 67 each. Ruangkit is a five-time winner on Asian Tour and a multiple winner on European Seniors Tour, while Tabuena, a silver medallist from 2010 Asian Games, has a professional best of fourth place at Taiwan Masters in 2011.
India's Rai said, "I think I played well. Controlled the shots well. I put the ball in play today and that's what I was planning on doing. I stuck to the game plan and it worked out. That's pretty much how you need to approach this course.
"You have to be patient, more than normally. Golf in general, you have to be patient but this course requires tremendous amount of patience and a solid game plan."
"I worked with my trainer, worked on my fitness. I played the US Open qualifying at Walton Health and was just practising and working on the basics. I wanted to work on a few areas that my coach and I wanted to focus on and felt that we could get better through fitness," he added.
The Santiburi Samui Country Club is one of the top golf courses located on the holiday isle of Samui and has steep uphill and downhill slopes. The winding fairways cut into the coconut forest as golfers battle the steep terrain.
Kiradech Aphibarnrat of Thailand, Korea's Baek Seuk-hyun and Rattanon Wannsrichan were tied for third with rounds of 68 each.
