Rory McIlroy admitted he felt “a little rusty” after he began his warm-up for next week’s US PGA Championship by finishing five shots behind round one leader Keith Mitchell at the Truist Championship in Philadelphia.

McIlroy – last year’s winner and a four-time champion at the event – recorded six birdies and two bogeys to card a four-under-par 66 at the Wissahickon Golf Course.

The 36-year-old, who completed a career Grand Slam with his thrilling win over Justin Rose at last month’s Masters, ended the day tied in 25th alongside nine other players.

“I was a little rusty out there, just hadn’t played in a week and I hadn’t really done a ton of practice,” said McIlroy.

“I drove the ball pretty well. I certainly can tidy a few things up, but overall a decent day.”

All four of McIlroy’s victories in this event, formerly known as Wells Fargo, were won at Quail Hollow, which hosts next week’s US PGA.

McIlroy said this week he has “nothing but positive vibes” as he approaches the second major of the year, having ended his long wait for a first title at Augusta National, and fifth major overall.

He finished tied 12th at last month’s Zurich Classic in his first outing since the Masters.

American Mitchell tops the leaderboard having posted a nine-under-par 61 – one stroke ahead of compatriot Denny McCarthy.

Mitchell parred the first five holes but got better as the day wore on with a run of four straight birdies coming towards the end of his round.

Ireland’s Shane Lowry is at six under, while Aaron Rai of England is at five under par.