The 30-year-old American had a bogey-free round and sunk five birdies on each of the front and back nine to lie clear of Australia's Aaron Baddeley and compatriot Bryce Molder.
There were nine players left out on the course when the opening day's play was halted by poor light, while 103 of 122 players who completed were on par or better.
It was the best round of Castro's career and he told PGA Tour Radio: "It was a solid round, all 18 holes. I made a lot of putts but they were all in that eight to 10-foot range so not gimmes but a lot of good looks."
Castro's round was capped by a birdie on the 220-yard par-3 seventh hole and he added: "That was the one bonus birdie today. It was a three-iron. That's just a hole you're trying to make par and scoot to the next tee.
"I played well for a couple of weeks during the Web finals and Frys. And Vegas I also played solid, I just didn't get any results out of it, so hopefully (I'm) just trying to turn the corner here."
Baddeley had six birdies and an eagle on the par-4 15th and said: "Overall, I'm very pleased and feeling very comfortable with where I'm at."
Canada's Adam Hadwin and England's Brian Davis are a shot further back on Baddeley and Molder on seven under.