Castro carded a stunning nine-under-par 63 on Thursday that put his name alongside Greg Norman and Fred Couples as sharing the Stadium Course record.
But the satellite tour graduate came back to earth in the second round with a six-over 78 that left him eight strokes behind halfway leader Sergio Garcia of Spain.
"It's fun. It's a lot of excitement," said the 27-year-old American. "The Players Championship is huge all over the world, so it's exciting. It's a good place to be.
"I've played plenty of Friday rounds, three or four outside the cut, and that's no fun. Today was fun."
Castro got off to a solid start when he birdied the par-five second hole, but he ran into trouble with a rough stretch that started at the par-four fourth and included four bogeys over the next five holes to set his slide in motion.
Castro, 267th in the world golf rankings, said he did not get down on himself for faltering in the second round.
"I just tried to be nice to myself," said Castro. "If it was easy, everybody would do it. I wouldn't have been embarrassed or felt bad or anything if I would have shot two 76s this week. It's a hard golf course.
"I'm three-under-par and beating 110 of the best players in the world through 36 holes. So I just kept telling myself, 'it's not easy. It's not easy. It's not easy.' I'm excited about the weekend."
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