Starting his round five shots off the lead, Panuphol or "Coconut" quickly hauled himself back within one shot of the lead with birdies on the second, fourth, sixth and seventh holes.
He dropped a shot on the eighth but regained his advantage with further gains on nine, 11 and 15 to sign for a six-under-par 66 and an eight-under-par 136 total at the Lake Karrinyup Country Club.
Fox, who missed out on the match play stages after losing in a play-off in 2017 and finished a shot out of the play-off 12 months ago, is poised to make his Sunday appearance after he birdied two of his last three holes to get to grab a share of the one-shot lead.
Pieters, who won the ISPS Handa Melbourne World Cup of Golf for his country last November, bounced back from the sick bed to move atop the leaderboard with a 66.
American Berry Henson is also hoping to feature in the shootout round after carding a 67 to stay two shots back from the leaders in a share of eighth place.
The top 65 and ties players after 36 holes will advance to Saturday where the leading 24 players after 54 holes of stroke play will progress to the six-hole knockout match play on Sunday.
The weekend cut was set at one-under with 66 players making it into the third round.