A 5-under-par 67 in Maryland gave Percy a one-shot margin over South Africa’s Keith Horne as he attempts to win his first over-50s major.

Percy revealed post-round that his physiotherapist had advised him to rest pre-tournament, restricting his preparation to a full course walk around Congressional with his caddie, and playing nine holes in the rain on Wednesday.

“On Sunday, it was so wet and I slipped on a rock and made a mess of myself,” the Victorian said.

“I’ve been in the physio Monday, Tuesday for a good hour, hour-and-a-half and then yesterday for another hour. I’m pretty sore right now.

“I didn’t have a practice round, so I was still trying to work out where to go on the course and that, so it was pretty tricky.

“In the weather we had today, I’m pretty happy with a 5-under. It was nasty out there for a while.”

Percy’s bogey-free round was highlighted by an eagle at the par-5 sixth, his 15th hole of the day, where his 4-iron approach finished inside a metre from the cup.

He then moved past Horne into the outright lead with a birdie at the par-4 eighth.

Percy admitted post-round it was the remarkable start by his playing partner, Denmark’s Soren Keldsen, that had helped him to his lead.

“I was really just trying to keep up with Soren. He started incredible,” Percy said.

“He started eagle-birdie-birdie, and could have birdied a few more holes.

“He sort of dragged me along to start with. He was playing so nicely, it definitely helped.”

The next best Australians are Stuart Appleby and Scott Hend, who are in a big group tied for seventh at 2-under-par.

Appleby, who had five birdies in his round, was 3-under before he dropped a shot at the closing hole, while Hend picked up four birdies mixed in with two bogeys on the back nine.

The Senior PGA Championship will be broadcast live on Fox Sports and Kayo, starting at 3am on Friday and Saturday and 5am Sunday and Monday.

Australasian scores

1 Cameron Percy 67

T7 Stuart Appleby 70

T7 Scott Hend 70

T24 Brendan Jones 71

T24 Richard Green 71

T24 Mark Hensby 71

T35 David Bransdon 72

T35 Rod Pampling 72

T35 Mick Smith 72

T48 Andre Stolz 73

T71 Michael Wright 75

T71 Steven Alker 75

T107 Michael Campbell 77

Photo: PGA of America