|Houston Open first round leaderboard|
|-9 S Piercy (US); -7 A Cejka (Ger), JB Holmes (US) -6 L Guthrie (US), P Mickelson (US), C Howell III (US), S Stefani (US).|
|Selected others: -4 P Casey (Eng), K Day (Eng); -3 J Rose (Eng).|
Three-times Masters champion Phil Mickelson warmed up for next week's opening major of the year with an impressive six-under 66 in the first round of the Houston Open on Thursday.
The American is joint fourth, three behind compatriot Scott Piercy, whose bogey-free 63 tied the course record.
JB Holmes and Germany's Alex Cejka lie joint second, a shot behind Piercy.
Paul Casey and Kelvin Day hit four-under-par 68s with fellow Englishman Justin Rose one shot further back.
Mickelson, who won this tournament in 2011, hit seven birdies in 17 holes after starting at the 10th but was denied a bogey-free round by a four on the par-three 9th.
The 44-year-old said: "It was a good round, a good start to the tournament. I got off to a quick start, birdieing three of the first four holes, and kept it going."
Piercy, who last won on the Tour at the Canadian Open in 2012, missed just one green in regulation and says his confidence grew as the round progressed.
"It's been building. I kept hitting good shot after good shot. By the end, I was [only] trying to make birdies," he said.
The Masters, in Augusta, starts on 9 April with Mickelson looking to add to the trophies he won in 2004, 2006 and 2010.