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.
Wisconsin business offering 'therapeutic cuddling' forced to close
Video of Victoria’s Secret models trying to 'twerk' hits Instagram