The big-hitting Kiwi carded a 63 to equal the course record from last week at Al Hamra Golf Club and sign for his lowest ever DP World Tour round to par at nine under.
Portuguese Ricardo Gouveia was his nearest challenger, with English pair Richard Bland and Andrew Wilson, Scots David Drysdale and Robert MacIntyre, Italian Francesco Laporta, Dutchman Joost Luiten, German Yannik Paul, Australian Jason Scrivener and Swede Sebastian Soderberg all three shots off the lead.
Fox graduated from the European Challenge Tour in 2016, the same year he represented his country in the Olympic Men's Golf Competition, and he became a two-time Olympian at last year's Tokyo Games.
The 35-year-old secured four Rolex Series top tens in his first two seasons on Tour before winning the 2019 ISPS Handa World Super 6 Perth and is now looking for a second DP World Tour victory.
He missed the cut after struggling with a back injury last week but looked to he showing no ill effects as he carded ten birdies with a single dropped shot.
"It was nice to get in the zone, I just felt like I had control of the golf ball," he said.
“I probably need to throw a couple of thanks out too, I struggled last week here with my back and I have my physio out here, and the Tour physio has helped me out over the weekend and I feel 100 per cent this week which was nice.
“Shaun Norris was nice enough to give me a putting tip on Tuesday when we played a practice round together and that seemed to work pretty well too so big shout-out to those guys.
“I hit it in the greenside trap a couple of times and got up and down. It is the kind of golf course where you feel like you’ve got a lot of opportunities and it was nice to take advantage of those today and then hole a couple of longer putts on some of the tougher holes to keep the round going.
“I worked hard today on just focusing on every shot, obviously my body felt a bit better today than last week, I felt like I could do what I wanted to do and I wasn’t fighting it so hopefully I wake up tomorrow feeling good and I can keep doing what I’m doing.”
It was nice to get in the zone, I just felt like I had control of the golf ball
Fox made a birdie from 21 feet on the tenth and then holed from similar range on the 12th and got up and down from the sand at the driveable par-four 13th.
A lovely pitch at the 16th set up another gain and while he gave the shot straight back, he made another up-and-down from the sand to take advantage of the par-five 18th and turn in 32.
More excellent bunker play brought a birdie on the driveable par-four first and when he two-putted the par-five third and put an approach to five feet at the fifth, he was alongside Gouveia.
A 30-footer on the sixth edged him into the solo lead and a fourth birdie of the day from the sand on the eighth gave him a cushion.
Gouveia also matched his lowest round on Tour with a 65 containing eight birdies and a bogey.
The 30-year-old is back on Tour after graduating from the Challenge Tour and turned in 31 as some dialled-in iron play brought birdies at the first, third, fourth, sixth and seventh.
He dropped a shot at the tenth but two approaches inside ten feet brought back-to-back birdies on the 12th and 13th before he made an excellent recovery from a ragged tee-shot for a final gain of the day on the 17th.
Luiten and Scrivener were both bogey-free in their rounds, while Bland, MacIntyre and Wilson all dropped a single shot, with Soderberg making two bogeys.
Paul holed out from 101 yards on the first for one of two eagles, Laporta also made an eagle as he holed a bunker shot at the 13th, and Drysdale birdied four of his last five holes having been one over after six.