|Houston Open second-round leaderboard|
|-12 A Putnam (US); -11 P Mickelson (US), A Cook (US); -10 G DeLaet (Can), L Guthrie (US)|
|Selected others: -7 P Casey (Eng), K Day (Eng); J Rose (Eng)|
Phil Mickelson was one stroke behind leader Andrew Putnam after the second round of the Houston Open.
The three-time Masters champion carded seven birdies in a five-under-par 67 that left him tied for second with fellow American Austin Cook.
Compatriot Putnam, 26, went round in 65 to take the lead on 12 under in Humble, Texas.
Englishmen Kelvin Day, Paul Casey and Justin Rose are tied for 18th, five shots behind Putnam.
Mickelson, who won the event in 2011, will compete in the Masters at Augusta - the first major of the year - starting on 9 April.
Putnam, who has not won on the PGA Tour, made a solid start before six birdies in his last seven holes saw him claim the lead from Mickelson and Cook.
"It's been a little bit of a struggle recently," Putnam said. "I've been working hard to get back and working on my swing."
Mickelson, 44, who began the round in fourth, said: "My short game has been sharp this week.
"I've hit some good pitches. The putter does feel a lot better. I'm excited about this weekend."