Connect to share and comment
Chapchai, who has gone without a win for four years, fired eight birdies and two bogeys at the Delhi Golf Club while the 20-year-old local hope Joshi carded nine birdies on his professional debut.
The pair hold a one-shot lead over Chan Kim of the United States, while India's Chiragh Kumar and Bangladesh's Siddikur made 68s at the $300,000 event.
Chapchai is hoping to add to his four Asian Tour wins, the last of which came in 2009.
"My putting was very good. I made a lot of one putts, probably took 27 or 28 putts," the 29-year-old said according to a press release by tournament organisers.
"I stuck to my game plan. It's a tough course as the fairways are narrow. But if you hit fairways, you can make birdies."
For professional debutant Joshi, victory on Saturday would mean a stunning start to his career.
"It will mean the world to me if I can win my first Asian Tour tournament as a pro. I'm looking forward to it," said the slim player from Bangalore.
Scores after round 1 of the SAIL-SBI Open being played at the par 72, 6,935 Yards Delhi Golf Club course
66 - Chapchai Nirat (THA), Khalin Joshi (IND)
67 - Chan Kim (USA)
68 - Chiragh Kumar (IND), Siddikur (BAN)
69 - Namchok Tantipokhakul (THA), Shiv Kapur (IND), Jason Knutzon (USA), Abhishek Jha (IND), Manav Jaini (IND), Carlos Pigem (ESP)