(Reuters) - Brooks Koepka did not get the luxury of seeing the full Trinity Forest layout before the opening round of the AT&T Byron Nelson in Dallas on Thursday but that did not keep him from shooting a six-under-par 65.
Koepka, who next week will defend his PGA Championship title, saw the back nine during a Tuesday practise round but never got a chance to see the front nine as rain washed out the pro-am event on Wednesday.
Undaunted, the world number three still compiled eight birdies in the morning wave on Thursday morning and trailed American leader Denny McCarthy by two strokes with half the field back in the clubhouse.
"It's a bit different when you're playing it blind," Koepka said, according to PGATour.com. "Kind of don't know where you're going. Relying a lot on the (yardage) book. I never look at the book."
Koepka, winner of three major titles in the past two years, is using the Byron Nelson to tune up for Bethpage next week, his preparation in contrast to most of the big names who are taking the week off.
The early starters caught a good break with the weather, playing in nearly still morning conditions before a breeze sprang up and made for a sterner test for the afternoon wave.
Late starters included Jordan Spieth, Hideki Matsuyama, Henrik Stenson and former Dallas Cowboys quarterback Tony Romo.
McCarthy's 63 was even better than it might look at first glance, because 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.
Australian world number 22 Marc Leishman withdrew before the round, citing a back injury.
(Reporting by Andrew Both in Cary, North Carolina; Editing by Frank Pingue)