Percy had nine birdies against a pair of bogeys while Romero had an eagle, seven birdies and two bogeys in taking a one-stroke lead over Blayne Barber and Jon Curran.
Nine other players, including Angel Cabrera, are tied at 5-under.
Percy rode a hot putter into the lead, totaling just 27 putts on the day and 10 on the back nine. He opened his day with back-to-back birdies and, after dropping a shot at the par-4 fifth hole, birdied three of his next four to make the turn in 4-under.
He had a bogey at the 10th but dropped putts for birdies at 12 and 13 and added others at 16 and 18.
Romero, starting on the back nine of the Trump International Golf Club Puerto Rico, was 4-under through his first 10. He had an eagle at the 547-yard, par-5 second hole and birdies on three of his next four holes in getting to 9-under. However, bogeys at 7 and 8 brought him back to the field and a share of the lead.