Tied for second were fellow American Bryce Molder and Australian Aaron Baddeley, who both carded 64s, one stroke better than Englishman Brian Davis and Canadian Adam Hadwin.
Baddeley and Davis both chipped in for eagle at the short par-four 15th, which played at 317 yards.
Molder reached seven-under with his birdie at the 10th but managed just two more birdies the rest of the way.
"I was thinking 'let's keep the pedal down' and had some opportunities but they didn't fall," said Molder.
Bunched at six-under 66 were Canadian Graham DeLaet, Andres Romero of Argentina, Venezuela's Jhonattan Vegas and Americans Andrew Loupe and Michael Kim.
First-round play was suspended due to darkness with nine players still on the course.
While more celebrated players were contesting the World Golf Championships-HSBC Champions event in Shanghai, Castro took aim at his first PGA win at an event that has crowned eight first-time winners including former world number one Luke Donald in 2002.
Castro, the nephew of former LPGA Tour player Jenny Lidback of Peru, won attention on tour two years ago when he shot an opening day 63 at The Players Championship to equal the course record shared by Fred Couples and Greg Norman.
Castro, whose 62 on Thursday marked a career low, also had his best career finish on the PGA Tour a month later when he finished runner-up to Bill Haas at the 2013 AT&T National.
(Editing by Steve Keating.)
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