Norton - Jon Rahm fired a five-under 66 on Saturday to take a two-shot lead at the Dell Championship as world number two Jordan Spieth and former US Open winner Justin Rose charged up the leaderboard.
PGA Tour newcomer Rahm, who is ranked fifth in the world, shot an opening round 67 for a total of nine-under 133 at the halfway stage of the second event of the 2017 PGA Tour playoffs.
Canada's Adam Hadwin (65), Englishman Paul Casey (65) and Americans Kyle Stanley (68) and Kevin Streelman (65) are tied for second at seven under at the TPC Boston course.
Phil Mickelson (67) is in a group of four tied for sixth at six-under 136, one shot ahead of Spieth and Rose, who both rebounded from one-over 72s in the first round to shoot 65s on Saturday.
Both of Rahm's mistakes came on his front nine which he began on the 10th tee. After parring his first two holes he made bogey on the par-four 12th.
The 22-year-old Spaniard had a double bogey on the par-three 16th but answered it two holes later with an eagle on 18. His 200 foot approach set him up nicely for a 12-foot putt that curled right to left into the hole.
"It felt like very different - that front nine and back nine," Rahm said. "I played a nearly perfect back nine. Felt like I didn't miss a shot.
"But that front nine is the one that probably looked worse than what it felt. It felt like I was making great swings. We misjudged the wind a couple times or we got a couple gusts and ended up missing greens."
Dustin Johnson, who won last week and took a one stroke lead into the second round, cooled off Saturday with a one-over 72 that included two double bogeys on the back nine. The 33-year-old American is tied for 14th, five shots adrift of Rahm.
While world number one Johnson's game went south, Spieth and Rose are trending upwards with identical scores for the second straight day.
Spieth, who lost in a playoff to Johnson last week, started his round the way he finished with a birdie. He also birdied the par-five No 2.
On the back nine he had just one bogey on 16 but sandwiched it between birdies at 15 and 17.
"I just woke up on the wrong side of the bed yesterday," Spieth said. "I woke up and got out here, and I was just cranky.
"For me it was just a matter of just stop being cranky."
Rose's game also intensified as the round progressed as he birdied five of his seven final holes.
"Always nice to respond with a few birdies. Keeps you moving forward," he said.
Defending champion Rory McIlroy shot a three-over 74 in the second round for a two-day total of four-over 146 to miss the cut by one stroke.
McIlroy will still advance to next week's BMW Championship despite missing the cut as the defending champ for only the second time in his career.
Leading second-round scores on Saturday in the Dell Technologies
Championship, second event of the PGA Tour playoffs (USA unless noted):
133 - Jon Rahm (ESP) 67-66
135 - Adam Hadwin (CAN) 70-65, Paul Casey 70-65, Kyle Stanley 67-68, Kevin Streelman 70-65
136 - Grayson Murray 68-68, Phil Mickelson 69-67, Bryson DeChambeau 69-67, Marc Leishman (AUS) 67-69
137 - Hudson Swafford 69-68, Justin Rose (ENG) 72-65, Kevin Na 68-69, Patrick Cantlay 69-68, Jordan Spieth 72-65