More than two-thirds of the 156-player field broke 70 over the three courses used for the tournament with Castro and Hahn tied for the top at 14-under 130.
Castro shot a 67 over the PGA West Palmer Course and Hahn, trying to become the second rookie in as many weeks to win on the tour, also had a 67 at the LaQuinta Country Club.
Phil Mickelson, playing for the first time this year, shot a 67 at the PGA West Nicklaus Course for a 139 total, but that was good only for a tie for 91st place.
Mickelson will need a low round Saturday to make the 54-hole cut. The survivors will then play the final round Sunday over the Palmer Course.
This tournament is known for its low scores, a tradition highlighted by the 59 David Duval shot in the final round to win the event 14 years ago.
2014: The Year in Music [PHOTOS]