Rookie Russell Henley also put himself in contention to win for the second straight week on a typical day of low scoring in the California desert, but Phil Mickelson struggled in his 2013 debut.
Hahn shot a 9-under 63 on the PGA West Palmer Course, one of three used for this week's pro-am event. That put him in a tie for the top spot with Roberto Castro and Jason Kokrak, neither of whom has come close to winning on the tour.
Castro shot his 63 on the PGA West Nicklaus Course and Kokrak played at the LaQuinta Country Club.
Hahn, 31, an American citizen born in South Korea, failed in his original bid to make it onto golf's big stage.
He went to work with an advertising agency, earned his real estate license and then sold shoes for six months before using the money he saved to try to get back on the tour. He did so by finishing fifth on the Web.com money list last year.
Henley, who won in Hawaii last week during what was his debut as a tour member, shot a 64 at the Nicklaus Course to tie Aaron Baddeley, Greg Chalmers and Doug LaBelle for fourth place. Henley's round was highlighted by an eagle putt from the fringe of the green on the par-5 fourth.
Mickelson had a poor day with the putter and settled for an even-par 72 that left him in a tie for 122nd place in a field of 156. The cut will come on Saturday after everyone has played each of the three courses. The final round on Sunday will be held at the Palmer Course.
Swim Week Miami Beach 2014 [PHOTOS]