It was a zone few have ever encountered, and it led to an odd potential problem.

Dustin Johnson nearly made too many birdies.

The defending FedExCup champion, Johnson likes to replace his golf ball after each birdie, and his caddie/brother, Austin, keeps 15 new ones in the bag to start each round. Fair enough. But when D.J. went 11 under through 11 holes in the second round of THE NORTHERN TRUST at TPC Boston last year, well, simple math suggested there might be trouble ahead.

“I was getting a little nervous,” Austin says. “Luckily, he had two eagles in there, so we only went through nine balls through 11 holes. If we ever run out, I keep the old ones in the bag, so if he somehow makes 15 birdies, we’ll just have to recycle one. I’d love to have that problem.”

Welcome to the “Peak D.J.” experience. It’s mesmerizing. Awe-inspiring. And perhaps untouchable.

Johnson won THE NORTHERN TRUST by 11 shots last year, and although the host course will be Liberty National this year, those who were at TPC Boston won’t soon forget it.

The opening round was blah, as Johnson made nothing on the greens and shot 67, which left him three back of leaders Daniel Berger, Cam Davis, Harris English, and Kevin Streelman.

In the second round, though, Johnson started birdie, eagle, birdie, eagle, birdie. At 7 under, it was the best first five holes anyone had ever played in the ShotLink era. He cooled off, sort of, but was still 11 under through 11, the best opening 11 holes of a round since the PGA TOUR implemented its first electronic scoreboard system in 1983.

“He was going crazy,” said Marc Leishman, one of his playing partners for the first two rounds. “It looked like he was going to shoot 55. He kept getting a new ball from his caddie because he changes his ball after every birdie, and I asked him, ‘How many golf balls do you carry?’”

Those watching the round went into 59 watch. Leishman wondered if Johnson might eclipse the TOUR’s lowest-ever round, Jim Furyk’s 58 at the 2016 Travelers Championship.

“It was one of those rounds where you’re just there watching,” said Abraham Ancer, the other member of the threesome. “I was cheering him on. I wanted to see history. He made a putt on 12 or 13 for par, and it was a tough one. He had a chance to shoot whatever, 56 or something like that. It was really, really cool to watch somebody like that just not miss.”

Johnson eventually ran out of magic, his bid to join the elite 59 club taking its last big blow as his drive went too far and his ball settled down in a grass swale at the par-5 18th hole. He hacked out, hit the green in regulation, two-putted, and signed for a 60. He wasn’t disappointed.

For 61% of the round he’d been positively electric.

“He did everything absolutely perfect for those 11 holes,” Austin Johnson says. “Hit it great, putted it great, and didn’t even really have to chip, I don’t think.”

Adds Dustin, with characteristic understatement, “I just did everything well.”

Scottie Scheffler actually did shoot 59 Friday, so the tournament wasn’t over. He and Johnson had each scorched the course, and with Johnson two ahead, they were paired together Saturday. Both came back down to earth in the third round, but in Johnson’s case, only briefly.

“I don’t think he was even playing that great,” Scheffler says. “He was like 4 under or something with a few holes left, and I remember he finished birdie, eagle and shot 64.”

Johnson, the 2011 and 2017 NORTHERN TRUST champion, looked destined to do it again, as he was five ahead of Scheffler and English going into Sunday.

It now fell to English to go toe-to-toe with peak D.J.

“I went into the last round like, OK, I’m going to put some pressure on him,” English says. “But I was 3 or 4 under through eight, and I had lost ground. There’s not many guys where if they’re clickin’ and I’m clickin’ I feel like they have an advantage. But with him hitting it 10, 15 yards past me every time, then he’d hit it 10 to 15 feet closer than me, and then he was putting really well, it was like, this guy doesn’t miss shots. At all. He was hitting his driver so well.

“It was like, I can do what I can do, but this guy is on a different level,” he continues. “It’s incredible to watch; you have to play your game and let him do what he does. It’s different, but when Dustin gets in a zone like that it’s probably as close to Tiger as a guy has been out here.”

Johnson signed for a final-round 63. His 30-under total tied Jordan Spieth (2016 Sentry Tournament of Champions) for the second-lowest score to par in a four-round TOUR event. It was the lowest score to par and lowest total in the history of the FedExCup Playoffs.

Was it perfect golf? Not quite, thanks to his so-so first round. Could anyone have beaten him?

In this era, Tiger Woods’ best golf was the closest to flawless, says Paul Casey, who finished T49 that week. Rory McIlroy and Johnson are somewhere just below that, he believes.

“Dustin, his swing isn’t as gracious as Rory, and he doesn’t walk the same, but my God is it efficient,” Casey says. “Nothing is wasted.”

Johnson was 23.53 total strokes better than the field average, the third time he has gained 20 or more strokes en route to victory. Only McIlroy (four times) and Woods (13 times) have more TOUR wins while outperforming the field by 20 or more strokes in the last 37 years.

“He’s so golf-savvy,” Casey says of Johnson. “He doesn’t get enough credit for that. He understands the spin rates and yardages, hits fades and draws – his golf-craft is just sublime. Dustin in full flow, you feel like there will be no blemishes. It’s clinical. It’s ruthless.”

Johnson himself doesn’t go that far. Still, he allows, “For four days it was definitely by far the best everything has worked.” As for potentially running out of balls, he smiles and shakes his head. “No,” he says, “I keep enough that I won’t do that.”