Jordan Spieth fired nine birdies to reach 12 under par at The CJ Cup Byron Nelson, while Scottie Scheffler also enjoyed a strong second round, but both trail runaway leader Si Woo Kim after two rounds at TPC Craig Ranch.
Kim's stunning second round of 60 saw him join Blades Brown and Andrew Putnam to equal the lowest score on the PGA Tour this season, with the South Korean moving to 18 under par heading into the weekend, and five shots clear of a chasing pack of players, which includes world No 1 Scheffler.
His five-stroke lead is one shy of the tournament's 36-hole record, set by defending champion Scheffler last year, with Kim hitting 12 birdies, but he agonisingly finished his second round with a bogey at the 18th.
Playing in his home state of Texas, three-time major winner Spieth started his back nine - the front side of TPC Craig Ranch - with six consecutive birdies on the way to a nine-under 62 in the second round. Spieth's blitz on the back nine matched his record for the most consecutive birdies in a round on the PGA Tour.
The 32-year-old credited a crucial par save at the 18th for setting up his birdie binge, which started with a 12-footer. He hit the first four fairways after struggling off the tee on his first nine, made a 12-footer at the fourth, a four-footer on the fifth and capped off the surge from nine feet on the sixth
"I thought making the par on 18, hitting the fairway on one, I just hadn't found any fairways," Spieth said. "When that happened, I was like, all right, I should get some wedges coming up. My putting feels good today. Made some nice adjustments last night.
"I've been working hard on it, so it's not just a one-click thing, but I felt like I got a nice feel that it allowed me to be more outwardly focused."
While Spieth was unable to complete the career Grand Slam with victory at the PGA Championship last week and has also failed to finish inside the top 10 at a tour event this season, the American is buoyed by where his game is trending.
"It's physically and my mechanics. I've been trying to build it back, and then I'd compensate and do what worked," he added.
"This last off-season, I said no more compensating because, to be consistent, I've got to get it back to a certain place, and it's been work from then to try to get there. It's all in mechanics and health."
Scheffler started at five under in an afternoon pairing with Brooks Koepka, who began the week strongly on Thursday, posting a first round score of 63, to move one shot off the leader, Taylor Moore, heading into the second round.
On Friday, Koepka, who is looking for his first victory since his return to the PGA Tour from LIV Golf, shot a round of 69 to move back to 10 under par as Scheffler climbed 14 places to move to eight under par, with a second round of 63.
Sungjae Im climbed 45 places up to tied second on the leaderboard after posting a second round of 61 that included an ace, as well as an eagle and seven birdies.
It was his lowest score on the PGA Tour in 752 rounds, and it could have been lower, with Im's only blemish coming on his ninth hole, the par-four 18th, with the South Korean finding a fairway bunker on the right of the fairway, before going down for a bogey.
He bounced back with birdies on his 10th, 12th and 14th holes, before going four under par through his final four holes.
After Spieth put his tee shot 29 feet right of the pin to the far left of the seventh (his 17th hole), his playing partner Im bounced his 222-yard shot between the fringe and the hole and watched it roll in. The 28-year-old was tied for the lead at that point.
"That was one of the prettiest hole-in-ones I've ever seen," Spieth said. "Prettier than any one I ever made. There are only a few people who would land that left of it on purpose. He might be one of them."
Spieth's birdie run ended when his 29-footer at the seventh came up about a foot short, while Im added another eagle with a 14-foot putt at the ninth.
Spieth finished a bogey-free round with a birdie on the ninth hole.
"I was hurting my head trying to figure out what our best ball was," Spieth said. "I think it was 57, which is pretty good."
Spieth, who contended on Sunday as a 16-year-old high schooler at his first Nelson in 2010, finished fourth a year ago when Scheffler won by eight at 31 under while tying the tour's 72-hole scoring record of 253.
The 32-year-old, who has 13 victories on tour, was actually worried about the cutline when he was five under at the turn. Then he went on to match his career best at the Nelson from the final round last year.
"Just a lot of it just comes down to knocking in putts," he said. "It's not a normal week, and it's fantastic. Now that our families have grown and stuff, you get the little kids coming out, and it makes it even better."
Watch the The CJ Cup Byron Nelson live until Sunday on Your Site Golf, where Scottie Scheffler is the defending champion. or .
