|Texas Open first round leaderboard|
|-5 Charlie Hoffman (US) -4 Aaron Baddeley (Aus) -3 Max Homa (US) -2 Phil Mickelson (US) Ryan Palmer (US)|
|Selected others: +1 Brian Davis (Eng) +2 Retief Goosen (SA) Marc Warren (Sco) Shane Lowry (Ire)|
Graeme McDowell's preparations for the Masters suffered a setback when injury forced him to withdraw from the Texas Open midway through the first round.
The Northern Irishman injured his left leg during practice on Wednesday and pulled out on Thursday after playing the first nine holes in five over par.
McDowell, 35, tweeted: "With The Masters upcoming (9-12 April) getting fit is imperative."
American Charley Hoffman leads by one after a five-under-par opening round.
McDowell, who won the 2010 US Open, had received physiotherapy ahead of his first round at the TPC San Antonio course and played with his leg strapped up.
The world number 23 is scheduled to compete in the Houston Open next week.
McDowell had been playing with Americans Dustin Johnson and Jim Furyk on Thursday when he advised them he could not carry on.