Ben Martin shot a 5-under 67 on Friday to maintain a course-record pace at TPC Louisiana, keeping him atop the leaderboard through two rounds of the Zurich Classic.
After breaking the course record with a 62 on Thursday, Martin opened the second round with two birdies and an eagle on his first four holes.
He shook off a bogey and double bogey on successive holes in the middle of the round and birdied four of his last nine to move to 15 under - the best 36-hole total at the course.
No one has had a four-round score better than 20 under.
Andrew Svoboda shot a 68 to remain second at 12 under, but closed his round on the par-3 ninth with his first bogey of the tournament.
Sueng-Yul Noh and Robert Streb were 11 under, and Erik Compton and Peter Hanson were another stroke back.