Percy had nine birdies and two bogeys at Trump International Golf Club while Romero mixed two bogeys with seven birdies and an eagle to set up a one-stroke lead over Monday qualifier Jon Curran and sponsor invitee and fellow American Blayne Barber.
Two-time Major winner Angel Cabrera was among a group of nine players a further shot back at five-under 67.
Starting on the back nine, Romero was on fire and at nine-under through 15 holes before bogeys on the seventh and eighth dropped him back a few pegs.
"I had a good feeling about my game, and today I just felt confident on each and every shot I hit," Romero said. "I felt I couldn't miss, and thank God, I'm putting well again."
Percy started on the first and opened with back-to-back birdies to get things going and finished strong with four birdies in his last seven holes.
The Australian's round featured just 27 putts, 10 on the back nine, all the more impressive given he used a new putter after his son had his usual putter early in the week, forcing a change as he searches for a maiden win.
"My son had my putter last week and I didn't have a putter this week until about 4 p.m. yesterday afternoon," said Percy.
"About 2 p.m. the Ping guy was on the green and I went over and tried a few putters out. I had about 15 minutes, went bang and tried a new putter out, and it felt really nice and I rolled the ball beautifully so I kept it."
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