The 2013 Malaysian Open champion double bogeyed the par-five hole and dropped another shot at the last to sit 12-under after his round of 68.
"I've been striking the ball so well in the first three rounds. It's a good way to start," said Aphibarnrat.
"First, I made three straight birdies to jump up at the top. I was still hitting the ball great on the back. Then I made a big mistake on last two holes, not the way to finish."
Halfway leader Peter Uihlein lurked two shots behind after birdying the penultimate hole for the third time this week to return one-under-par 71.
"I lost the focus on the short putt on 17," conceded Aphibarnrat.
"That is the big turn, the big key for me after Peter made birdie, and we swung three shots on that. That's a big move. But I made a mistake and I lost some concentration."
Uihlein was equally unhappy with his round.
"I just made too many sloppy bogeys. Four bogeys, a bit rough, five birdies and missed a couple pretty easy ones," rued Uihlein, who has not won in European Tour since his maiden triumph at the 2013 Madeira Islands Open.
Spaniard Pablo Larrazabal carded the joint lowest round of the day, a seven-under 65, to lie further one shot behind while two-time Masters winner Bubba Watson was two-over after carding his second successive round of 74.
(Reporting by Amlan Chakraborty; editing by Sudipto Ganguly)
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