Jordan Spieth shoots 62 but has uphill climb at his hometown Byron Nelson after Si Woo Kim's 60

Spieth started his back nine — the front side of TPC Craig Ranch — with six consecutive birdies on the way to a 9-under 62 and was 12 under.

Spieth was one shot behind playing partner Sungjae Im, who aced the par-3 seventh moments after Spieth's sixth birdie in a row and finished his 61 with an eagle on the par-5 ninth. Im was tied with Scheffler, Japan's Kensei Hirata and Wyndham Clark, who matched Scheffler's 63.

A nearly $25 million renovation at the TPC Craig Ranch added bunkers and put plenty of contours in the greens. But a rainy Thursday and minimal wind left the Lanny Wadkins-led redesign as vulnerable as the course was the first five times it hosted the Nelson.

“I think the problem right now is that this is like the first time in the history of Dallas, Texas, that you’ll have four or five days of very little east wind and soft conditions in May,” Spieth said. “You get your normal what we had on Monday out here, that’s how it’s designed. I think it would show that it’s significantly harder, but also fair.”

Tom Hoge shot 62 and Tony Finau had a 63 to join Spieth at 12 under, one stroke ahead of first-round leader Taylor Moore, who followed his opening 62 with a 69. Tyler Duncan and Keith Mitchell also were at 12 under after each shot 66.

Brooks Koepka, who opened with a 63 and is looking for his first victory since his return to the PGA Tour from LIV Golf, shot 69 while playing with Scheffler and Kim. He was eight shots back.

Spieth credited a par at No. 18 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 par-3 fourth, a 4-footer on the par-5 fifth and capped the surge from 9 feet on the sixth.

After Spieth put his tee shot 29 feet right of a pin to the far left at No. 7, Im bounced his 222-yard shot between the fringe and the hole and watched it roll in, raising his arms and looking skyward after the ball dropped. 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’s only a few people that would land that left of it on purpose. He might be one of them.”

Spieth's birdie run ended when his 29-footer at No. 7 came up about a foot short, while Im added another eagle with a 14-foot putt at the par-5 ninth. Spieth finished a bogey-free round with a birdie on No. 9. Im had a bogey and seven birdies to go along with his hole-in-one and eagle.

“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 Spieth with 13 tour victories was actually worried about the cut line when he was 5 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.”

Scheffler kept himself in contention by playing a four-hole stretch in 5 under on the back nine, including a 45-foot eagle putt on the par-5 12th.

Hirata, a six-time Japan Golf Tour winner looking for his first PGA Tour victory, followed a 64 that included an eagle with a bogey-free 65. A short birdie putt on the ninth pulled him even with Im.

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