Ruangkit and Robert Thompson stand at 7-under-par 137 for a one-shot advantage over Rod Spittle.
An Asian Tour staple, Ruangkit was through 15 holes and held the lead alone at minus-eight when play was suspended due to darkness Thursday evening. He shot an even-par 72 in the second round.
Thompson, who has never participated in a Champions Tour event, posted a five-under 67 on Thursday to earn his share of the lead.
The 36-hole cut fell at 12-over 156, knocking out just four players based on scores. One player withdrew and another was disqualified, leaving 85 for the weekend.
The third round began at 9:30 a.m. (ET) with golfers teeing off in threesomes at Nos. 1 and 10.
Spittle, who earned $36,528 in three Champions Tour starts this year, including the U.S. Senior Open, shot a 1-under 71 in the second round and is alone in third place at six-under 138.
Graham Banister managed only an even-par 72 and is tied for fourth place with John McGough, who carded a second-round, 3-under 69. The pair is knotted at 4-under 140.
The top-30 players and ties this weekend will become eligible to compete in qualifiers held for open, full-field Champions Tour events next season.