The world number two ground out a two-over-par 72 at treacherous Oakmont Country Club, finishing six strokes off the early pace set by fellow American Andrew Landry at the weather-hit major championship on the outskirts of Pittsburgh.
"It was tough, starting and stopping and conditions changing from when we started to when we finished, but it is what it is," Spieth told reporters after carding two birdies and four bogeys in a round interrupted three times on Thursday by thunderstorms.
"Bit of a shame, because Oakmont, it was so great in those practice round days when it started to firm up. Now, you know, it's still great, but it's a different golf course.
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"Overall, I felt like I played well. I felt like I didn't quite get rewarded with my score for how I felt like I played. A couple tough breaks. It's a U.S. Open. I'm still in it."
Spieth, who won last year's U.S. Open at Chambers Bay by one stroke, said that approach shots had become a very different proposition at Oakmont with the firm conditions from earlier in the week having been softened by rain.
"It made the back pins actually much more challenging, I thought, after the delays," said the 22-year-old Texan. "Those front nine back pins, I hit a couple shots that I thought were perfect and all of a sudden they just ripped back.
"I hit good shots, had a couple bad swings. But for the most part, I played like an even-par round, and you'd take four of them here. Just didn't quite get the couple shots that I thought I deserved, but sometimes you gain a couple you didn't deserve."
Spieth will not start his second round at the weather-delayed event until Saturday when he could end up playing 36 holes in one day, a prospect that left him unperturbed.
"You're talking guys that are playing 36 today and possibly us playing 36 tomorrow? I really don't mind that at all," he said. "You stay loose. You just stay in the same rhythm.
"Obviously, you hope you get in a good rhythm to start that you can kind of gain momentum off of."
(Editing by Larry Fine)
