"Anytime you have back-to-back par 5s that are reachable, it's definitely something to look forward to," Hahn said. "Especially if the round's not going well."
Castro had the lead alone at 16 under, but bogeyed two of his last three holes three-putting the par-4 ninth on PGA West's Arnold Palmer Private Course.
"A couple slipped away there at the end, but yesterday I made a 50-footer on the last," Castro said. "Today, I felt like I hit a good putt and three-putted. So, that's stuff over 72 holes that's going to even out."
Hahn opened his rookie season on the tour last week in Hawaii with a tie for 67th in the Sony Open. The 31-year-old South Korean-born American played briefly at the University of California and won the Web.com Tour's Rex Hospital Open last year in a playoff.
"I'm just soaking it in, having a good time," Hahn said. "Anytime that I play a good round, it feels good and makes me cherish the momentum a little bit more, because I know they're few and far between."
David Frost of South Africa had a 7-under 65 Friday to take a one-stroke lead after the first round of the Mitsubishi Electric Championship in Kaupulehu-Kona, Hawaii.
The tournament opens the 34th Champions Tour season at Hualalai Golf Club.
The field of 40 has champions from the past two years and major champions from the past five, plus eight sponsor exemptions.
Frost made a 40-foot eagle putt on the seventh hole to reach 5 under. His only bogey came on the next hole, and three birdies on the back nine put him into the lead.
Roberto Castro -14
James Hahn -14
Darron Stiles -13
Scott Stallings -13
Richard H. Lee -13
Daniel Summerhays -11
Phil Mickelson -5