The 19-year-old Korean, who opened with a 68 at the Rayong Green Valley Country Club, stumbled with a triple-bogey on the second hole but recovered strongly with five birdies on his way home for a two-day total of five-under-par 137.
Yoon made the grade in his first attempt at the Asian Tour Qualifying School last year but failed to retain his card after finishing 131st on the 2017 Order of Merit.
He hopes to play his way back onto the region's premier Tour by breaking into top 35 and ties after the intense 90-hole battle this week.
Minkyu Kim, also of South Korea, fired seven birdies against two bogeys for a 66 to grab the clubhouse lead on 136.
The opening round of the Qualifying School Final Stage was earlier disrupted by a lengthy rain suspension on Wednesday which resulted in a total of 123 players returning to complete their rounds on Thursday morning.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
