Long-hitting Justin Thomas, co-leader overnight, fired a five-birdie 67 at the TPC Deere Run to take over on his own at the top, with fellow Americans Johnson Wagner (63) and Tom Gillis (65) a further stroke back at 11 under.
Spieth, who took time off after his U.S. Open win at Chambers Bay last month, was delighted to rebound from an opening 71 where his short game was especially rusty.
“Today was a big step forward,” said the 21-year-old, who is using the tournament to tune up for next week’s British Open where he will aim to add the third leg of a grand slam of the majors.
“I was not feeling great after Wednesday (the pro-am), and then after yesterday … but to come out today and shoot a solid round when I know I needed to and to have my putter working when I knew I needed it to gives me a lot of confidence.
“I putted about as well today as I have the past couple months,” said Spieth, who totalled just 25 putts in the second round. “Ultimately this was a big day for me in my preparation for next week.”
Some have questioned the plan by Spieth to arrive at St. Andrews just three days before the start of the British Open but the American is happy with his preparations for the year’s third major.
“I got a good amount of practice in and I’ll be able to adjust to the speed and the conditions there,” he said. “I feel like I’ll be able to do that in a couple days.
“It is a career goal of mine to have a career grand slam (of the majors). If it happens in one year, that would be pretty special. I’m just focussed on St. Andrews.”
Rookie Thomas, seeking his first PGA Tour victory, was “very excited” to be leading after 36 holes.
“I stayed really patient, I kept hitting my lines and my birdies were pretty low-stress,” said Thomas. “I’m in a good spot.”
The cut fell at four-under 138 with former major winners Lucas Glover, Retief Goosen, Trevor Immelman and David Toms among those in failing to advance.
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Greg Stutchbury)