The 36-year-old, who has two PGA Tour wins to his name, is two shots clear of compatriot JB Holmes and Germany???s Alex Cejka after a blistering nine-birdie round at the Golf Club of Houston.
He began his round on the 10th tee and made a steady start with birdies at the 10th and 13th, but he stormed up the leaderboard with five straight birdies from the 18th and extended his hot streak by picking up further shots at the sixth and ninth to equal the tournament course record. ????
Piercy was among the early starters and led by three for much of the day before Holmes and Cejka signed for 65s in the evening.
Cejka had a chance to claim a share of the lead when he arrived at the ninth on eight-under, but he then dropped his first shot of the round with a bogey four and had to settle for a share of second place.
Three-times Masters champion Phil Mickelson looked sharp in his preparation for next week???s season-opening major at Augusta National as he carded a six-under 66.
Mickelson, the 2011 champion in Houston, blazed through his first nine holes in five-under 31 on the way to his lowest round since January.
He chipped in on his first hole, the short par-four 10th, hit a driver on the 338-yard, par-four 12th hole about 20 yards short of the green and chipped close for birdie, then nearly holed a bunker shot at the par-five 13th for another easy birdie.
The left-hander got to seven-under on the front nine before finishing with a bogey after he failed to get up-and-down from a bunker at the ninth.
“My short game actually was what threw away so many shots last week. I spent the last three days working on that,” said Mickelson, who finished tied for 30th at last week’s Texas Open.
“So I was able to turn that around in a few days and the game feels pretty good.
“This is a big week for me. I felt the game was close last week. The only thing missing was my chipping and short game.
“If I can play well this week, it should be some good momentum for next week.”
Mickelson is alongside fellow Americans Luke Guthrie, Charles Howell III and Shawn Stefani on six-under.
Frenchman Victor Dubuisson and Spain???s Sergio Garcia both made solid starts with five-under 67s, while Englishman Paul Casey, the 2009 champion, opened with a 68.