The Australian registered birdies on six of the first 12 holes on his way to an eight- birdie, one-bogey round at Gleneagles Golf Course on Thursday.
He has claimed a one-shot lead in the PGA Tour Champions' final major of the year over France's Lionel Alexandre.
"I drove it really good, but you have to here," said Percy, who found something that clicked late in his practice round on Wednesday. "I didn't hit in the rough once and so that was really nice."
Alexandre, 53, does not have a ton of experience playing in majors - a 72nd-place finish at the 2000 Open Championship serving as his only major during his PGA Tour days. But it didn't affect Thursday as he notched birdies on six of the final 12 holes for his 66.
"Today it was really, really cool," Alexandre said. "My caddie is a really good friend, we joke a lot on the course. I just take it as a regular tournament on the Tour. I see the same guys. Maybe some ... a few more names than are playing in the States."
South Africa's Ernie Els leads a quartet sitting at five under. While Els enjoyed a relatively quiet round with six birdies and one bogey, Tom Pernice Jr. sprinkled an eagle and a double bogey into his 65. Pat Perez and Spain's Miguel Angel Jimenez also posted 65s.
"I had not played much this year," Perez said, who enjoyed a bogey-free trip. "This is only my ninth tournament round. Any time I play, I start off-season again, so I never really know what's coming. I prepared the last two weeks."
Jerry Kelly tops the group of five golfers at 66 while 11 more carded a 67 on a day when the links-style course was dry and relatively tranquil.
Defending champion Padraig Harrington of Ireland, who struggled with knee pain the previous two weeks while failing to make the cut at the Scottish Open and Open Championship, was not among the 44 golfers in the 144-man field to break par.
Harrington settled for a 70 as he matched three birdies with three bogeys, but he reported no issue with his knees.
"I had an excellent day with both of my knees," Harrington said. "I had a lot of trouble in Scotland, affected me playing golf a little bit yesterday and the day before, but today, it's fine."
Australian Michael Wright shot a four-under 66 and is tied seventh, while compatriots Richard Green, Steve Allan, Greg Chalmers and Mark Hensby are all on one under.
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