“You feel like you can shoot a score in the morning and the greens are perfect, that if you do roll it well, you feel like you hole everything.”
After six birdies in the opening 10 holes, Fox’s shot at a historic 61 was undone by a bogey at the 13th.
He bounced back with three birdies in the final five holes to climb into contention going into Sunday’s final round.
Fellow New Zealander Kazuma Kobori also had a strong day, shooting a three-under 67 to move to four-under and a share of 12th with plenty of players still on the course.
A leaderboard packed with some of golf’s biggest stars is primed for a spectacular weekend in pristine conditions on England’s northwest coast.
Cameron Young, world No 4, begins his third round at six-under.
World No 1 and defending champion Scottie Scheffler is two shots further back but well in the running at four under, alongside home favourite Tommy Fleetwood and two-time major winner John Rahm.
Rory McIlroy’s hopes of making a charge up the leaderboard appeared to be fading as he was one over for the day through eight holes to slip to level par for the tournament.
But a monstrous drive and chip in for eagle at the par-four ninth brought the Masters champion back within six shots of the lead at the turn.