He opened the AT&T Byron Nelson with a 6-under 64 to share the first-round lead with Ricky Barnes, but Hahn dropped six strokes behind Jason Kokrak after an even-par 70 on Friday.
So Hahn simply went out on Moving Day and fired another bogey-free 64 to grab a one-shot lead over Billy Horschel heading into the final round.
“Anytime you're under par after nine holes you're kind of freed up a little bit, and I feel like I did that today,” Hahn said. “Made some good decisions. Wasn't too aggressive with my approach shots.