Just as Herbert, the overnight leader by two shots, had equalled the lowest round ever recorded at a men's major championship on Friday, Fox became the third man at Royal Birkdale this week to shoot a landmark eight-under round of 62 on Saturday.
Fox's early brilliance meant that by the time Herbert was the last to tee off, he had already been caught at eight-under by his trans-Tasman colleague.
Then as 'moving day' unfolded on the Lancashire links, 30-year-old Herbert was initially knocked out of first place as his pursuers began to make a move, only to regroup after dropping a shot at the third hole to reach the turn in one-under and move back to the head of the leaderboard at nine-under.
As he moved into the inward half of his round, the Australian LIV star was one shot clear of Fox and Sam Burns, the other member of the '62 club' who'd also shot an eight-under round on Friday.
Just a shot behind them were charging local hero Tommy Fleetwood, Korean Si Woo Kim, Cameron Young and Herbert's US playing partner Jackson Suber, who were all on seven under going into the day's business end.
Fox's remarkable move on Saturday had rocketed him from level par to joint-leadership at eight under alongside Herbert, still an hour-and-a-half before the Aussie was due to tee off.
There were great similarities between the brilliant efforts of 39-year-old Fox, son of the All Blacks World Cup great Grant Fox, and Herbert on Friday as they putted the lights out on the Lancashire links.
Fox made birdies at the second, third, fifth, sixth and eighth, completing an outward nine of 29, which was just one shot more than the record-breaking 28 recorded by Herbert on Friday.
The Aucklander then made another at the 10th and, despite his only bogey of the round at the 13th, responded immediately to birdie the 14th, 16th and 17th to give himself a chance of the first ever 61 if he could birdie the last.
A poor drive into a fairway put paid to that hope, but Fox played one of the shots of the week to blast out some 180 yards, even while clipping the top edge of the sand trap, to find the 18th dance floor.
From 47ft away, he putted up short to five foot but, unlike Herbert 24 hours earlier, was able to sink his par-putt.
It earned him congratulations from his playing partner Xander Schauffele, another of the men who's shot a 62 in men's majors.
Light winds prompted some early low-scoring on Saturday with Schauffele having moved up to four under after his 66, but things really started getting tight when the leaders started going out with 13 players still separated only by four shots as the round was coming towards its conclusion.
Rory McIlroy was unable to make much headway, ending up on two under after a 69 and he admitted he was probably "a shot or two" out of contention.
Meanwhile, all eyes were on Bryson DeChambeau as he returned to action after the two-stroke penalty imposed on him following his second round on Friday, when R&A officials deemed he had inadvertently improved the area of his intended swing when he had been marching around in the long grass on the fifth hole.
DeChambeau, three shots behind Herbert going into the round, was level for the day after 12 holes, having been given a warming reception from his army of fans.
