It was not Tiger Woods but the guy playing with him: Bryson DeChambeau, who is coming off a four-shot victory in the first FedEx Cup playoff event and shot 63 while playing with Woods for the first time in a tournament.
DeChambeau and Tyrrell Hatton (69) were one shot behind.
It figures to be another frantic Labor Day finish on the TPC Boston.
Ancer, at No. 92 in the FedEx Cup and just hopeful of getting into the top 70 to advance to the playoff event next week outside Philadelphia, was at 13-under 200. A victory would assure a trip to the Tour Championship and a spot in the Masters.
Even with 18 holes left, that's a long way off.
Ten players were within four shots of the lead, a group that included Justin Rose, Rory McIlroy and Jordan Spieth.
Woods had to settle for a 69 that left him in a tie for 16th, six shots behind. At least he saw plenty of good golf. DeChambeau has become one of his regular practice partners, though they had never been in the same group until Sunday.
DeChambeau opened with two birdies and finished even stronger, with short birdie putts on the 16th and 17th and then a 5-iron from 237 yards that stopped rolling 2 feet from the hole for an eagle.
"He's my childhood idol," DeChambeau said. "I've admired him my whole entire life. And to be finally able to play with him under tournament conditions, it was different. I was a little nervous, for sure. But I was able to get out there and execute shots and worry about my game and focus on hitting the best shots possible. And I was able to play really well like that."
Not since Vijay Singh in 2008 has anyone captured the opening two FedEx Cup playoff events. DeChambeau led by four shots at Ridgewood Country Club last week and never was seriously challenged. This time, the 24-year-old Californian will have to come from one shot behind against a 27-year-old playing in only his 49th PGA Tour event.
Hatton had the lead until his approach on the par-4 12th went off a cart path and into the trees, and a search party of some three dozen fans and volunteers never found it. He did well to escape with a double-bogey and then finished with a birdie for a 69.
Justin Rose (70) and Cameron Smith (67) were another shot behind, followed by Emiliano Grillo (64) and Kyle Stanley (66).
McIlroy was making a charge until hitting into the water on the 16th hole and missing a short putt at the 18th. He had to settle for a 68 and was in a group at 9-under 204 that included Beau Hossler (68), Ryder Cup hopeful Tony Finau (67) and Spieth (68).
On another gorgeous day that allowed for good scoring, Spieth didn't make a birdie until the seventh hole, dropped two shots on the tough stretch of the back nine, answered with two birdies and at least gave himself a chance.
Spieth has yet to win this year, and with two events left, he has not secured a spot in the Tour Championship.
More than being six shots behind, Woods has to climb past 15 players. He handled the par 5s but didn't make much else and spent some 45 minutes on the putting green after his round.
"I didn't get a lot out of my round today," Woods said. Looking ahead to Monday, he said he would need a round like what he saw from DeChambeau.
"I've got to make a bunch of birdies, get off to a quick start and just get rolling early," Woods said. "Kind of do what Bryson did today."
Ancer knows what it's like to be among the chaotic atmosphere that Woods brings to a tournament. He played in the group ahead of Woods in the third round of the Quicken Loans National, heard the cheers and delivered a 62 that hardly anyone saw.
He didn't play his best from the final group, but it wouldn't have mattered with Francesco Molinari closing with a 62.
Ancer got hot again, putting for birdie on every hole except No. 12, on which he made a mental mistake by hitting 3-wood off the tee into a breeze and into a bunker. He was able to advance only 100 yards on the 51-yard par 4.
Even with seven birdies and a 65, Ancer missed three birdie chances inside 10 feet, including a 5-footer on the par-5 18th.