Romero, the Argentine player who won the PGA Tour's 2008 New Orleans event and the European Tour's 2007 Deutsche Bank tournament, had an eagle and seven birdies on his first 15 holes to reach 9 under, then fell back with the bogeys on the par-4 seventh and par-5 eighth.
Percy, from Australia, had nine birdies and two bogeys on the Trump International course.
Blayne Barber and Jon Curran were a stroke back, and two-time major champion Angel Cabrera opened with a 67 to match Bill Lunde, Steve LeBrun, Brad Fritsch, Justin Bolli, Morgan Hoffmann, Brian Stuard, Peter Uihlein and Rafael Campos. Patrick Cantlay, the 20-year-old former UCLA player coming off a Web.com Tour victory last week in Colombia, shot 70.
Romero has missed the cuts in four of his five starts this year, shooting 75-67 last week in the Honda Open.
"I'm happy about the way things went," Romero said. "Last week on Thursday I started striking the ball quite well and then played really well on Friday, but it wasn't good enough for me to make the cut. I was very pleased at the end, and I practiced a lot over the weekend. I had a good feeling about my game, and today I just felt confident on each and every shot I hit. I felt I couldn't miss, and thank God, I'm putting well again."
Percy, a five-time winner on the Australasian Tour, tied for 71st in the Honda Classic, wasting a 71-66 start with weekend rounds of 77 and 78.
"I just drove the ball really nicely," Percy said. "I've been trying to not hit the ball too hard, so I just hit three-quarters drivers and just kept the ball in play. I did that today and it was quite nice."
Cabrera played alongside countryman Romero and Colombia's Camilo Villegas.
"I had a really good time with them," Cabrera said. "They are really good friends, so we enjoyed a lot and had lots of fun out there."
Villegas shot 71. Defending champion George McNeill also had a 71.
South Korea's Si Woo Kim withdrew after a 79. The 17-year-old Kim, playing on a sponsor exemption, earned a tour card at the qualifying tournament, but won't become a full tour member until he turns 18 in June. He missed the cut at Pebble Beach in his only other tour start of the year.