World No.22 Marc Leishman has been forced to miss the Byron Nelson, the lead-in event to the US PGA Championship.
Runner-up last year, Leishman withdrew from the Dallas tournament because of a back injury.
"My lower back just flared up a bit on Monday night and Tuesday," Leishman told pgatour.com.
"I think I slept on it a bit funny.
"I was hoping it'd be good by today because I really love the tournament but it wasn't feeling too flash this morning so I decided to withdraw and seek some more treatment."
A four-time PGA Tour winner, including this season's CIMB Classic, Leishman is hopeful he can take his place in next week's US PGA Championship at Bethpage Black in New York.
"I'll get it more thoroughly checked out and worked on but I hope it just needs some rest and I'll be OK for next week," he said.
In Leishman's absence, Matt Jones led the way for the Australian contingent with an opening round six-under-par 65.
Starting on the back nine, Jones' round included an eagle on his 15th hole – a 301-yard par-4.
He drove the green before sinking a nine-foot putt.
RIGHT: Jones is two shots behind the leading Denny McCarthy at Trinity Forest. PHOTO: Michael Reaves/Getty Images.
Leader Denny McCarthy's 63 was even better than it might look at first glance.
He double-bogeyed his fourth hole before roaring back with 10 birdies for a one-stroke advantage over fellow Americans Tyler Duncan and Tom Hoge.
The links-style Trinity Forest layout is designed to be played in firm conditions with a prevailing southerly breeze.
But conditions on Thursday were quite the opposite. Persistent rain over the past week has left the course unusually soft while the breeze was from the north.
Brooks Koepka did not get the luxury of seeing the full Trinity Forest layout before the opening round but that did not keep him from shooting a six-under-par 65.
Koepka, who next week will defend his US PGA Championship title, saw the back nine during a Tuesday practice round but never got a chance to see the front nine as rain washed out the pro-am event on Wednesday.