Stenson, 38, won his last tournament on the European Tour, retaining the DP World Tour Championship in Dubai in November, but his attempts to tweak his game at the National course proved disastrous.
"First hole of the round, I missed the drive, hit the cart path and finished up in a bush and ended up with a seven," Stenson told reporters.
"I just didn't convert any birdie chances, made zero birdies for the round. I had a few things I was working on with my swing and (they) didn't really work out that good either. I just didn't play that good at all."
The Swede dropped further shots at his 13th and 18th to be 12 strokes behind leader Martin Kaymer and in peril of missing the cut for the first time on the European Tour since Abu Dhabi a year earlier.
"When you haven't played competitively for four to five weeks you're always going to be a little rusty, but it's more a few things on the technical side I'm working at," added Stenson, who is aiming to win a first ever major in 2015.
"I'm trying to improve, get a few things in order and that just didn't work out that well."
(Reporting by Matt Smith; editing by Martyn Herman)
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