Nelly Korda admitted she was surprised how poorly she had played as she began her US Women's Open challenge with a two-over 73.
The world No 1 has finished no worse than tied-eighth in 10 of her last 11 worldwide starts, but has some work to do at Riviera Country Club if she is to make it back-to-back major wins after her Chevron Championship success.
Korda, who started on the 10th, looked on track with a birdie at the 11th but followed that with back-to-back bogeys at the 12th and 13th.
A second birdie at the first took the American back to level-par but further bogeys at the third and seventh saw her slip down the leaderboard.
"I honestly just hit it really poorly off the tee," said the 27-year-old, who finished tied for second behind Maja Stark in last year's US Women's Open.
"Found myself in a lot of trouble on the wrong sides of a lot of these pins. I just felt like I was grinding to make safe pars.
"It wasn't a great day. I hit it really good Monday through Wednesday, so I have honestly no idea where this came from. I'm going to go to the range.
"I've just been feeling really stuck in my swing and not being able to release it. I was sending it out right in Cincinnati [at the Kroger Queen City Championship] too. I put in a lot of hours on the range, and I can't seem to really figure it out."
Korda is seven shots behind the early pacesetter Jennifer Kupcho, whose previous major title came at the 2022 Chevron Championship, as the 29-year-old made seven birdies in a five-under-par 66.
She holds a one-shot lead over Sei Young Kim of South Korea, whose four-under 67 saw her start and finish strongly with back-to-back birdies at the 10th and 11th and then three successive birdies from the sixth on her 'back nine'.
"I was hitting the ball well today, also rolling the ball really well with the putter," said Kupcho. "So seeing a lot of things fall definitely helped with the confidence with my putter.
"I kind of just felt at home. I feel like I really like the golf course. It kind of just flies by. I don't know, I feel like I just really, really like the golf course.
"I've missed the last three US Open cuts. So barring something crazy doesn't happen tomorrow, I'll be playing on the weekend.
"I think it kind of just lets me relax and not have to worry about, OK, I just need to make the cut. Just go out there and play my best, take every shot as it is, and see what happens at the end of the day."
Kim, the 2020 Women's PGA Championship winner, is one ahead of her compatriots Hyunjo Yoo and Ina Yoon and Mexico's Gaby Lopez, who was six under after 10 holes before slipping back with three bogeys.
England's Lottie Woad and Charley Hull matched Korda's 73, while Leona Maguire of Ireland struggled to a five-over 76.
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