Kim's second straight bogey-free round left him on 13-under 127 after 36 holes at TPC River Highlands in Cromwell, Connecticut.
"Just not really thinking about the score," Kim said. "I'm playing really well and I know kind of what I'm doing on the golf course, so just really executing my game plan and the score is the score.
"I've been playing really solid. It's not about this week, it's about keep building these momentum blocks for the rest of the season."
World number one Scottie Scheffler, who celebrated his 28th birthday on Friday, closed with a birdie to share second on 129 with two-time major winner Collin Morikawa and fellow American Akshay Bhatia on a day where an afternoon thunderstorm halted play for three hours and 14 minutes.
Kim, who shared second at last year's British Open, finished a season-best fourth at the Canadian Open earlier this month and has not missed a tournament since the end of April.
"I think this stretch is making me more sharper and more ready and I think it's kind of time to show," Kim said.
World number 26 Kim, who shared 26th at last week's US Open, seeks his fourth PGA title and first since defending his crown at the Shriners Children's Open last October.
This is the last signature-level event of the PGA season, offering a limited field and no cut.
Kim made a birdie putt from just inside 17 feet at the second hole and added a tap-in birdie at the par-5 sixth after reaching the green in two.
Then he reeled off three birdies in a row starting with a 14-foot putt at the par-3 eighth, a putt from just inside 15 feet at nine and an eight-footer at the 10th.
Scheffler had a streak of three birdies just before the stoppage, making a four-foot birdie putt at the par-5 13th, another birdie from just inside 19 feet at 14 and a birdie putt from just outside six feet at 15.
He concluded a round of 64 by sinking a birdie putt from just inside eight feet at 18.
"After the delay I came out and hit some really nice shots," Scheffler said. "It was nice to birdie 18 to finish. Before that I feel like I played a lot of solid golf and did some good things, so overall it was I think a pretty solid day."
'Real steady'
Morikawa birdied six of the first nine holes on the way to shooting 63.
"It has been real steady," Morikawa said. "Shots are going where I want. Putting the ball in the fairway and that's key out here, especially with some nasty rough. Just got to continue that for the next two."
Reigning Olympic champion Xander Schauffele, who snapped a two-year win drought at last month's PGA Championship, was fifth on 130 after a second 65.
Scotsman Robert MacIntyre fired a PGA career-low 62 to stand on 132.
