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Fox had rocketed up the leaderboard after firing a sublime five-under second round 66 to offset his opening four-over 75.
The 30-year-old Kiwi climbed from 80th at Quail Hollow into a share of 11th at the halfway mark, before his third-round 71 left him tied for 12th with three others on Sunday (NZT).
Fox, ranked 101st in the world, mixed three birdies with three bogeys to sit one under the card overall, six strokes behind leader Kevin Kisner of the US.
Kisner followed up back to back 67s with a third-round 72, and holds a one- stroke lead over compatriot Chris Stroud and Japan's Hideki Matsuyama .
Kiwi Danny Lee missed the cut after recording 76 and 78 in his first two rounds and finishing 12 over the card.