Wetterich played the back side first on the Sunrise Course at Mirasol and hit a 5-iron to 10 feet for a birdie at the par-3 11th. Wetterich then wedged his third shot within 4 feet for a birdie at the par-5 12th to reach 8 under.
The 31-year-old knocked a 4-iron inside 12 feet for a birdie at the par-4 18th and reached the green in two at the par-5 fifth. Wetterich, who tied for eighth at this event in 2002, two-putted for another birdie and made it two in a row with a birdie at the par-5 sixth.
'I definitely have come a long way since even two years ago,' said Wetterich, who finished 10th on the Nationwide Tour money list last year to earn his card. 'Last year, I think I learned a little bit, just to be patient out there. I still get frustrated and aggravated at times, but I don't think it's as bad as it used to be.'
At the par-4 ninth, Wetterich landed his approach within 20 feet of the hole and drained the birdie putt to secure the 36-hole lead of a PGA Tour event for the first time in his career.
'It's nice, obviously, to be in a position where you have a chance to shoot a couple of good rounds and maybe change your life,' said Wetterich, who has missed three of four cuts this year. 'But I'm going to go out there and try to play and stay focused like I did the last two days and just take them as they come.'
Janzen also started on the back nine and promptly missed a short birdie try at the 10th. He responded with an 18-foot birdie putt at the 11th, but gave that shot back with a bogey at the 13th. Janzen then birdied the par-5 17th to make the turn at minus-4 before catching fire on the front side.
Janzen birdied the second and hit a wedge inside 8 feet for a birdie at the fourth. He ran home a 20-footer for an eagle at the par-5 fifth and two-putted for birdie at the sixth.
The 40-year-old dropped a wedge within 4 feet of the hole for a birdie at the seventh and played his tee shot to 25 feet at the par-3 eighth. Janzen then rolled in the birdie try for a round of 64.
'I came down here thinking I had the chance to play well,' said Janzen. 'I played well here last year. I didn't seem to really be doing anything until I made the turn. Once a couple of putts went in, they just started all going in.'
Campbell mixed three birdies and two bogeys over his first nine holes and a birdie at the par-5 12th got him within two of the lead. Campbell sank a long birdie putt at the par-4 14th to move to 11 under, but stumbled with two bogeys over his final four holes for a 71.
Jesper Parnevik, Brett Quigley and Patrick Sheehan tied for sixth at 7-under-par 137. Kevin Na followed at 6-under-par 138 along with Bob Estes, Billy Andrade, Pat Perez and Daniel Chopra.
Defending champion Todd Hamilton carded a 71 to finish eight shots off the pace in a group at 4-under-par 140 that features Vijay Singh, Jim Furyk and Davis Love III.
The cut fell at even-par 144 with 79 players qualifying for the weekend. Among the less fortunate were Bernhard Langer and Paul Azinger.