The Australian world number three backed up his opening 61 with an eight-under 63, including a 43-foot eagle putt at the last, to rocket to 18-under 124 in the third of the PGA Tour's four FedExCup playoff events.
"Usually it's hard to back up a big round and shoot a low one again but fortunately for us, we had calmer conditions today," Day told reporters.
"I feel like I'm in the zone. I feel very free and I feel like there's no stress ... there's obviously stress but I'm enjoying it. It's good pressure, and I'm loving it. It's a good position to be in.
"But there are 36 holes left and I have to regroup and come out ready again tomorrow."
Day tied the PGA Tour record low for the opening 36 holes as he built a commanding five-stroke advantage over Americans Brendon Todd and rookie Daniel Berger.
Todd holed out from the fairway for an eagle at the par-five 18th for a 63 of his own while Berger fired a 64 to post a 13-under total of 129.
World number two Jordan Spieth, who played with Day for the first two rounds, was a further two shots back after finishing birdie-eagle for a 66, level with fellow American Kevin Na (66).
Top-ranked Rory McIlroy was nine strokes off the pace in a tie for ninth after carding a 65.
PLENTY OF FIREWORKS
While plenty of fireworks went off on a day where the scoring average was a touch better than three under par, the biggest roars were reserved for the 27-year-old Day.
The in-form Australian, searching for a fourth victory in six starts, will become world number one for the first time if he wins this week and he relishes being the player to beat.
"I'm going to try not to back up. Sometimes when someone feels like they're getting the upper hand and you're able to roll a 20 or 30-footer in and they don't get any ground, I feed off that," said Day.
"It's a lot of fun to do that. Right now mentally I just feel confident with how I'm playing."
Masters and U.S. Open champion Spieth, who tried to mount a charge on Friday but fell further behind, could only applaud his opponent.
"I feel like I should be paying to come watch some of this. It was special," Spieth said.
"What he's doing right now on the course is something I haven't witnessed, watched or witnessed in my life. If he continues this pace, then he won't be caught. If he lets up just a little bit, then hopefully we can catch him."
Earlier on Friday, Day fell short in his bid for a record-equalling 59 in the weather-delayed opening round when he failed to hole out a 44-yard chip shot from the right rough, then missed a nine-foot birdie putt.
But the disappointment did not last long as he went back out and once again destroyed the Conway Farms course with seven birdies, an eagle and just one bogey.
(Editing by Mark Lamport-Stokes)
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