Ruangkit, a staple on the Asian Tour, stands at 13-under-par 203 and is five clear after three rounds at the TPC at Eagle Trace.
Robert Thompson shot a 1-under 71 and is alone in second place at minus- 8. Perry Arthur (67) and Rod Spittle (71) are knotted in third at 7-under-par 209.
After Saturday's final round, the top-30 players and ties will become eligible to compete in qualifiers held for open, full-field Champions Tour events next season.
The second round was completed Friday morning after darkness forced the suspension of play on Thursday. Thompson joined Ruangkit in the lead Friday morning, but Ruangkit cemented himself atop the leaderboard with his strong play early in round three.
Ruangkit birdied the par-3 third, then added back-to-back birdies at the fifth and sixth holes. He closed his front nine with a birdie at the par-4 ninth to reach 11 under par for the tournament.
Ruangkit gave one back with a bogey at the par-3 11th, but played spectacular golf down the stretch to all but seal a spot inside the top 30 after Saturday.
He birdied three in a row from the 14th to put serious space between himself and Thompson. Ruangkit is a five-time winner on the Asian Tour and in 1995, finished second on the tour's Order of Merit. His last victory on that circuit came at the 2004 Thailand Open.
'With very little wind today, I was able to stay aggressive and I'll try to do the same thing tomorrow,' Ruangkit said through his interpreter. 'I've been playing well lately, but I didn't think I'd have this big of a lead when I started out in the tournament on Wednesday.'
Thompson, who has never played in a Champions Tour event, made nine consecutive pars before a bogey at the 10th. He reclaimed the lost stroke with a birdie at 11, then traded a birdie for a bogey at 14 and 15.
He birdied the par-three 17th to move into second place by himself.
Doug LaCrosse posted a 4-under 68 and is alone in fifth place at minus-6, one shot better than Ray Stewart (69) and John McGough (71), who share sixth.
Graham Banister only managed an even-par 72 and has sole possession of eighth place at 4-under-par 212. Harry Taylor (66) and Greg Hickman (72) are tied for ninth place at minus-3.