Houston - Phil Mickelson, tuning up for a run at a fourth Masters title, had seven birdies on Friday but settled for a share of second place behind Andrew Putnam after two rounds of the Houston Open.
Putnam nabbed six of his seven birdies in his last seven holes, his seven-under 65 vaulting him to the top of the leaderboard on 12-under-par 132.
Mickelson's seven birdies were balanced by two bogeys in a five-under-par 67 that left him tied for second on 133.
He was joined on 11 under by unheralded American Austin Cook, who birdied his last four holes in a seven-under 65.
It was a further stroke back to Canada's Graham DeLaet, who carded a 67, and American Luke Guthrie, who signed for a 68 for 134.
Putnam, a PGA Tour rookie with just one top-25 finish on his resume, teed off on 10 at the Golf Club of Houston and was one under through nine holes.
But he caught fire coming in, stringing together four straight birdies starting at the par-four third.
"I caught fire on the back nine, started making a lot of putts," Putnam said. "These greens are so good out here, if you start rolling them on your line and seeing your line, you can make a lot of putts.
"I've played pretty good so far. I've kind of been a little loose with the driver, but this course is pretty wide open so you can do that."
He punctuated his round with two more birdies at eight and nine -- from the rough at the par-five eighth and with a 15-foot putt at the final hole.
Mickelson had set the early pace.
After going birdie-bogey at the second and third holes, Mickelson added three more birdies on the front nine.
His march continued with birdies at 11, 13 and 15 -- where he rolled in a 22-foot putt.
Mickelson's lead dwindled as a wayward tee shot cost him a bogey at 17, but he regrouped for a solid par at the last and held the clubhouse lead before Putnam stormed home.
"The golf course is in great shape as always. It's fun tournament, it's going to be a shootout. There's a lot of guys at eight-under, nine-under, 10-under par that are right there," Mickelson said.
Mickelson, who has yet to post a top-10 finish this season, is looking for a boost heading into the first major of the year at Augusta National next week.
He'll have plenty of competition on the weekend. Rising star Jordan Spieth carded a 66 on Friday and headlined a group of six players sharing sixth place on nine-under 135. He was joined by fellow American's Shawn Stefani, J.B. Holmes, Hunter Mahan and Mark Wilson and France's Victor Dubuisson.
Leading second-round scores on Friday in the US PGA Tour $6 million Houston Open (USA unless noted, par-72):
132 - Andrew Putnam 67-65
133 - Phil Mickelson 66-67, Austin Cook 68-65
134 - Graham DeLaet (CAN) 67-67, Luke Guthrie 66-68
135 - Shawn Stefani 66-69, J.B. Holmes 65-70, Jordan Spieth 69-66, Hunter Mahan 67-68, Mark Wilson 69-66, Victor Dubuisson (FRA) 67-68
136 - Keegan Bradley 70-66, Chris Stroud 68-68, Alex Prugh 67-69, Michael Putnam 68-68, Jonas Blixt (SWE) 68-68, Charles Howell 66-70
137 - Russell Henley 69-68, Alex Cejka (GER) 65-72, Nick Watney 69-68, Tony Finau 69-68, Kelvin Day (ENG) 68-69, Paul Casey (ENG) 68-69, Justin Rose (ENG) 69-68, Charley Hoffman 69-68, Scott Piercy 63-74, Johnson Wagner 69-68