The 21-year-old has plenty of work to do to make up ground at TPC Deere Run, where Justin Thomas and Nicholas Thompson lead after shooting eight-under 63s.
Spieth, who will pursue the third leg of the ‘Grand Slam’ at St. Andrews after winning the Masters and U.S. Open, said he had also struggled the previous day in the pro-am.
“Just a rusty round,” Spieth told reporters. “My first round of 18 holes since the U.S. Open was yesterday. I certainly saw a lot of rust yesterday and it was a slight improvement today.”
Spieth will probably have to get to around four-under to make the cut. He had three birdies and three bogeys in his first round on Thursday, missing several putts from inside 10 feet along the way.
“I’m going to have to do a little better than two-for-seven on up-and-downs in order to be able to play the weekend. It’s a little disappointing but at the same time I didn’t put in the preparation,” he added.
Spieth took a week off completely after winning the U.S. Open and said he probably should have spent more time on the course rather than the range when he got back to work.
“I came back and worked really hard with my instructor tee to green and putting but it was all range work,” he said.
“I didn’t get out on the golf course like I maybe should have, so my own course reps started yesterday here in preparation for this week and the Open.”
Co-leader Thomas is coming off a disappointing finish last week at the Greenbrier Classic, where he was in contention in the final round until a quadruple-bogey at the 16th hole.
“I’ve felt really good about my game the last couple weeks,” the 22-year-old Tour rookie said. “I’m probably going to learn a lot from a week like last week.
“As much as it sucks to finish what I did and be right there, it’s probably better for me in the long run, and I’m going to draw a lot more from that in a week where I get in contention.”
(Reporting by Andrew Both in Cary, North Carolina; Editing by Peter Rutherford)