Tulsa, Oklahoma - Rory McIlroy is riding solo atop the US PGA Championship leaderboard after Thursday's first round, shooting a five-under 65.

McIlroy was one under on the front-nine after birdies on two, five and nine, and bogeys on six and eight, followed by a clean back-nine that featured four consecutive birdies starting on the 12th hole.

He sits one stroke clear of Will Zalatoris and Tom Hoge as the only players at four under.

Speaking with Sky Sports after his round, McIlroy said he is enjoying the form he has been building since his final round at The Masters, as well as the joy of teeing off with such a decorated playing group.

"I have felt comfortable for the last few weeks," he said. "I took a lot from the final day at Augusta, and played well at the Wells Fargo.

"Once the draw came out I was excited to play in a group like that... I enjoy playing with Tiger [Woods] and Jordan [Spieth]. It's a pretty comfortable group for all of us... we all know each other pretty well."

"It was a great day, and great to get off to a good start in a major for a change. My game is in good shape, so it’s just a matter of executing the shots like I know I can."

Meanwhile, it was a tougher day for Tiger Woods, who finished four over after bogeys on three of the first four holes. Speaking to Sky after wrapping up, he said he was far from 100 per cent, but he is not throwing in the towel.

"Physically I have felt better - emotionally I’ve actually felt better too," he said. "It was frustrating."

"I got off to a great start... but I did not keep it going... that’s not normally how I play, but it’s how it was."

Justin Thomas birdied the 18th to grab a share of fourth, in a three-man group with Matt Kuchar and Mexico's Abraham Ancer at three under.

A number of exciting international contenders finished Thursday's round at two under, including Australian Cameron Smith, American Xander Schauffele, England's Matt Fitzpatrick, and the Chilean pair of Joaquin Neimann and Mito Pereira.

One further shot back at one under is last week's AT&T Byron Nelson winner Lee Kyoung-hoon, in a packed bunching including Tony Finau, Charl Schwartzel, and Talor Gooch.

Max Homa, Victor Hovland, and Shane Lowry highlight the logjam at even par, while some of the tournament favorites are at one over, namely Scottie Scheffler and Collin Morikawa.

John Daly was a pleasant surprise with a competitive score of two over, finishing tied with Spieth, Hideki Matsuyama, and Keegan Bradley.

Jon Rahm was disappointing at three over, as was Brooks Koepka at five over, and the ascending pair of Corey Conners and Patrick Cantlay at six over.