Japan's Hideki Matsuyama (68) and American Rickie Fowler (70) were tied in second at 10-under.
"I'm very confident with my putting at the moment and it seems like my ball-striking worked well today as well, so it was a fun day," Lee told PGATour Radio.
"My iron game's really good right now and I've been putting well since I switched to a claw grip (two years ago), so it seems like everything's going smoothly right now."
Bryce Molder (64), in a tie for fourth with fellow American Boo Weekley (65), stormed into the clubhouse with five birdies in the final seven holes in front of a raucous gallery, vaulting within four shots of the lead with the lowest round of the week.
Mickelson (65), who started the day a distant eight strokes off the pace, finished a bogey-free round in style with a 30-foot birdie at his final hole, the par-four ninth, to sit five strokes off the pace.
The man they will all chase on Sunday is 25-year-old Lee, who was born in South Korea before moving to New Zealand at the age of eight.
Lee was hailed in some circles as the next Tiger Woods when he turned professional in 2009 after winning the U.S. Amateur Championship the previous year.
He had already won on the European Tour as an amateur, but had a rough adjustment to the professional ranks and took a few years to make his mark after temporarily losing his way.
Lee posted his first PGA Tour victory last July at the Greenbrier Classic and has risen inside the top 50 in the world rankings.
Fowler, meanwhile, took consolation with being in striking distance despite struggling with the speed of the greens.
"Pleased to get that round out of the way," said the world number four. "I didn't have my best stuff out there, saw a lot of putts slide by.
"I was giving the greens a little too much credit and they were a little slow."
(Reporting by Andrew Both in Cary, North Carolina; Editing by Frank Pingue)
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