Tim Clark, J.J. Henry, Jim McGovern and Dean Wilson share second place at 6-under-par 64. Six players, including Justin Leonard, are one stroke further back at minus-5.
Purdy got off to a hot start on the Resort Course at LaCantera Golf Club. He parred the 10th, his first, then dropped a wedge within 10 feet at the 11th to setup his first birdie.
He came right back with a 3-foot birdie at the 12th and made it three straight when he stuck a wedge within 3 feet of the cup at 13.
Purdy wasn't done there. After he knocked his second shot over the green at the par-5 14th, he nearly chipped in for eagle, but settled for a kick-in birdie to quickly move to 4 under.
Purdy, a two-time runner-up this season, reeled off five consecutive pars around the turn. He hit a 7-iron to 2 feet at the second for his fifth birdie of the day.
He came right back with a birdie at the third to climb to 6 under. Purdy holed out from the fairway with a sand-wedge at the par-4 eighth for eagle.
'I hit the ball better today than I have maybe ever in my life, and I can attribute that to I had a great message last night from a guy named Jim Weathers,' Purdy said. 'I was relaxed and the club was on plane and I was hitting it far and square, and it was a lot of fun.'
At the ninth, his last, Purdy rolled in a 10-footer for birdie to tie the course record set three times previously.
'The last hole was a driver and if it hadn't been uphill, I probably would have hit that 400 yards,' said Purdy, who won once on the Nationwide Tour. 'That drive, I hit it straight down the middle. That was probably the longest putt of the day, 10 feet, which was a great way to end a great round.'
Henry also started on the back nine and birdied three of his first seven holes. He ran off four straight pars around the turn, before birdieing the par-3 third to move to minus-4. The 29-year-old then dropped in back- to-back birdies from the sixth to get to 6 under.
Clark, a two-time winner on the Nationwide Tour, got off to a hot start. He ran off five straight birdies from the par-4 11th. Around the turn, he birdied the first and fourth to climb to 7 under. The South African bogeyed the par-4 eighth to slip back into a tie for second place.
McGovern stumbled out of the gate with a bogey at the 10th, his first. However he came right back to birdie No. 11. He dropped in back-to-back birdies from the 16th and again from the first to climb to minus-4. The 1993 Houston Open winner converted birdies at the seventh and eighth to tie for second.
Wilson, best known as one of Annika Sorenstam's partners at the 2003 Colonial, began with four straight pars before carding an eagle at the par-5 14th. He came right back with a birdie at the 15th. The Hawaiian then birdied Nos. 17 and 18 to jump to 5 under.
Around the turn, Wilson tripped to a bogey at the par-4 fourth. He fought back with birdies at the sixth and seventh to move to 6 under. He parred his final two holes to remain there.
Leonard is joined in a tie for sixth place at 5-under-par 65 by Jay Delsing, Scott Simpson, Mark Wilson, Scott McCarron and Patrick Sheehan.
Duffy Waldorf, Jerry Kelly, Tom Lehman and Loren Roberts lead a group of 17 players one stroke further back at minus-4.
Tommy Armour III, the defending champion, shares 29th place at 3-under-par 67.