At one point, Scheffler shared the lead with fellow major winners Jon Rahm (six-under) and Bryson DeChambeau (five-under) but they both dropped shots late in their rounds while Scheffler finished with an eagle and three birdies in his last five holes.
Swede Alex Noren is second following a superb five-under 66, with Americans J.T. Poston and Davis Riley a shot back in third. Overnight leader Jhonattan Vegas, Si Woo Kim of Korea, Matthieu Pavon and Rahm are all at six-under.
An almost 10m birdie putt at the third hole saw Fox move into a share of fifth at five-under but he then missed a 1m putt for par at the par-three fourth, leading to a bogey.
A lucky bounce out of the bunker at the short par four seventh saw Fox’s drive land on the green. He faced an eagle putt that would have seen him join the lead but it fell short and he tapped in for birdie.
He got to six-under with a second straight birdie at the par-four eighth but followed that up with a bogey.
A third bogey of the day at the par-four 11th saw Fox drop back to a share of 14th at four-under and he hovered around that score for most of the back nine. He missed a short birdie attempt at the 15th and to make matter worse dropped a shot at the following hole.
Fox made birdie at the 17th after being closest to the pin for the day, landing his tee shot within a metre but finished with a bogey at the last.
A thunderstorm postponed the scheduled start and forced organisers to re-draw the groups into trios off the first and 10th tees.