NEWTOWN SQUARE, Pa. — This was a quintessential Scottie Scheffler day.

Difficult as it may be to fathom, you might have forgotten he was in the field Thursday as the PGA Championship was carrying on through its opening round at Aronimink Golf Club.

How can you forget the No. 1-ranked player in the world for the past three years, and a player who has carried that top spot into the past 12 major championships, was playing?

Preposterous, right?

But Scheffler was playing in the third-to-last group off the first tee, a 2:05 afternoon start. And long before he began his round, player after player took his best shot running to the top of the leaderboard.

For a lot of the day, those players who finished their rounds residing at the top of the leaderboard were not exactly household names.

Aldrich Potgieter, a 21-year-old long-hitting South African, posted a 3-under par 67 to lead. Stephan Jaeger, a 36-year-old journeyman from Germany who probably wouldn’t be recognized walking the streets of Munich, joined at 3-under.

Ryo Hisatsune, a 23-year-old from Japan who’s never won on the PGA Tour, finished 3-under, as did Min Woo Lee, a 27-year-old Aussie with one PGA Tour victory.

Later in the day, Alex Smalley finished tied for the lead at 3-under, as would 41-year-old Martin Kaymer — who hasn’t had a top-10 finish in a major championship in 10 years, spanning 23 majors.

That leaderboard could easily have been mistaken for one of those PGA Tour opposite-field events, not a major championship.

And then, sometime after 5 p.m., there he was.

Scheffler. Tied for the lead, where he finished the day, along with six other players.

Look out. Scheffler began the round hitting his first 12 fairways — the most he’s hit in a row to start a major in his career.

But wait, there’s more.

Scheffler’s kryptonite this year has been opening rounds. The first 18 holes of tournaments collectively have been an Achilles’ heel for him, entering the week ranked 77th in first-round scoring average (70.33) and leaving himself on the chase after Thursdays.

Scheffler has blamed those slow starts for him finishing tied for third in Phoenix (after a first-round 73), tied for fourth at Pebble Beach (after a first-round 72) and runner-up in his past three tournaments — the Masters (70), RBC Heritage (68) and Doral (71).

Well, he seems to have corrected that. Just in time as he tries to defend the PGA Championship title.

“Definitely the best start I’ve gotten off to this year, maybe besides American Express [which he won],’’ Scheffler said.

Because this is the way he operates, Scheffler did it so matter-of-factly, so quietly Thursday.

As stealth as a World No. 1 can be, that’s the essence of Scheffler. As quiet as it was, though, you better believe every other player took notice.

This is the 27th major championship of Scheffler’s career and the first time he’s ever had a lead or co-lead after the first round — a career first he said afterward he was “not aware’’ of.

Then he joked, “Is it really a lead when you’re tied with six or seven other guys? I guess it’s a co-lead, with a big emphasis on the ‘co’ part.’’

Scheffler is the third defending PGA champion to have at least a share of the lead after 18 holes of his defending year. The other two are Tiger Woods and Brooks Koepka, and both went on to repeat as champion.

Amid the lesser-known players on the leaderboard are some of the game’s biggest names.

Xander Schauffele, a former PGA champion, is 2-under, as is Patrick Reed, who’s grinding his way back to the PGA Tour after fleeing LIV Golf.

Jordan Spieth, in his 10th try to complete the career Grand Slam, is 1-under. So, too, are Jon Rahm; Koepka, a three-time PGA champion; two-time PGA winner Justin Thomas; and fellow former PGA winner Collin Morikawa.

Matthew Fitzpatrick, Justin Rose, J.J. Spaun, Rickie Fowler, Patrick Cantlay and Hideki Matsuyama, all formidable champions, are lurking at even par. Even rock-star club pro Michael Block is at even par.

“It’s a really tight leaderboard,’’ Scheffler said. “At this moment, it’s anybody’s tournament.’’

There are 12 former major champions within two shots of the lead, and 48 players within three shots.

Scheffler, though, is the player everyone is watching. As he should be.