Golf's second major is upon us, after the PGA Championship kicked off Thursday at Oak Hill Country Club in upstate New York.

Heading into Round 1, the last two winners of the Masters Tournament, Jon Rahm and Scottie Scheffler, had been equally favored to win the Wanamaker Trophy, at +750. A win in this tournament would be a first for either of them.

Rounding out the favorites heading into Thursday were Rory McIlroy (+1200) and then Xander Schauffele, Patrick Cantlay, Justin Thomas and Brooks Koepka all at +2200.

First-round play was suspended due to darkness on Thursday night, but enough players have concluded Round 1 that we can confidently make the observation that there have been some shakeups atop the leaderboard.

Scheffler has played to expectations with a great first round. But Rahm took a tumble, opening up the gates for some unexpected contenders.

Let's break down three takeaways from the first day of the PGA Championship. Thursday's full leaderboard can be found at pgachampionship.com.

Odds via DraftKings Sportsbook

Favorite Scottie Scheffler Had a Historic Round

Heading into Thursday, Scottie Scheffler had never had a bogey-free round in a major.

After he concluded play in the first round of this year's PGA Championship, that's no longer true.

The 26-year-old, who finished his day with a 67 (-3), tabbed three birdies and 15 pars but no bogeys for the first time in his 51 career major rounds.

To put that in context of a historic season overall, per ESPN Stats & Info, coming into Round 1 on Thursday, Scheffler had made bogey or worse on just 9.5 percent of his holes on the PGA Tour this season. That's "on pace to be the lowest bogey percentage in a season over at least the last 40 years."

If you had Scheffler winning it all this year to add the PGA Championship to his list of major wins, you've got to be feeling pretty good heading into Round 2.

But some other favorites heading into this week's tournament took a surprising stumble on Thursday...

What Went Wrong for Jon Rahm?

Thursday went about as expected for PGA Championship co-favorite Scottie Scheffler, but the same can't be said for the other player who had the best odds of winning the tournament, Jon Rahm.

The Spaniard shot a 76 (+6) to put himself out of the top 100. He had the unfortunate distinction of posting the worst opening-round score to par by a world No. 1 at the PGA Championship, per ESPN Stats & Info:

It was a shocking turn of events for the 2023 Masters champion. In fact, not to pile on, but it was also the worst showing by a regining Masters champion in his next major round since Bubba Watson in 2014, when he shot 76 at the US Open.

Rahm was in good company in finding Oak Hill challenging, but a double bogey at the seventh really cemented his poor showing and dug a hole that he'll find challenging to climb out of.

Everything's Coming Up Ryan Fox

Ryan Fox has had some major developments in his personal life in recent weeks, both welcoming a child and recovering from pneumonia.

And now the Kiwi has some major developments in his professional life to boot, playing to an unexpected tie for sixth place following the conclusion of Round 1.

Heading into this year's PGA Championship, Vegas had the 36-year-old way down in the opening odds to win, at +25000. But he took control of a challenging Oak Hill course to storm his way to contention atop the leaderboard heading into play on Friday.

Shooting 68 and finishing two under in Round 1, Fox has put himself in position to legitimately contend for the Wanamaker Trophy, if he can keep it up.

A victory in the PGA Championship would be the first major win of his career; his previous best finish in a major tournament was 16th in The Open Championship in 2019.