The blazing finish by the reigning PGA Championship winner vaulted him ahead of an impressive group at five-under that included compatriots Adam Scott and Marc Leishman, Sweden's Henrik Stenson and Americans Brendan Steele and Troy Merritt.
"I drove it on a string, and played the par-fives in five-under," said the long-hitting Day.
"It was one of those days where everything kind of went well," added the Australian, who at the par-four 15th punched a 150-yard shot under a tree branch that ran between greenside bunkers to finish pin high for his birdie putt.
The round could have been even more special for Day save for a wild drive out of bounds left on the ninth hole that led to a double-bogey.
The red-hot Scott, winner of his last two events, again showed command of the short stick since having to give up his anchored putter this year, saving par twice from five feet, once from 10 and once from 12.
"The course is perfect so I had to take advantage of the morning conditions," Scott told the Golf Channel. "I didn't play my best golf tee to greens, but I chipped and putted really well."
World number six Scott has been in a zone, following a second-place finish at Riviera with victories the next two weeks at PGA National and Trump National Doral before taking a one-week break.
Ten players were bunched two strokes off the pace after shooting four-under rounds of 68, including former U.S. Open champion Justin Rose, fellow Briton Paul Casey, Italy's Francesco Molinari and KJ Choi of South Korea.
Rose had a pair of eagles, including one at the par-four third, his 12th hole of the day, from 112 yards to compensate for a double-bogey on his second hole.
Two-time defending champion Matt Every shot a one-under 71 while world number two Rory McIlroy was tied for 107th in the 120-man field after a three-over 75.
(Reporting by Larry Fine in New York; Editing by Frank Pingue)
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