Patrick Reed holds the clubhouse lead at The Open, but described the difference between the front and back nine at Royal Troon as "David versus Goliath".
Reed shot an eagle on the par-four third and posted further gains on the fourth, sixth and seventh holes to reach the turn at five under.
A pair of bogeys thereafter stemmed his progress but he held on for a five-under 66 to top the leaderboard - Rory McIlroy coming in three shots adrift, Jordan Spieth five back and Jason Day at two over.
However, high scoring on the inward nine has proved a theme at Troon, and the American suggested overconfidence from a strong start could be an issue.
Asked to describe the difference between the front and back nine, Reed responded: "David versus Goliath.
"But it played pretty easy it felt like, especially on that front nine, as long as you hit some good tee shots. It doesn't matter if the wind's blowing or not blowing, that back nine is tough.
"It's one of these golf courses that it allows you to get off to a quick start and allows you to get almost overconfident and cocky. Then you get too careless on the back nine.
"You could go shoot 31, 41. It's one of those kind of golf courses. You have to stay humble on it, and you have to take your medicine if you hit a wayward shot on strategising to at least salvage bogey."
The last six Open Championship winners at Troon have hailed from the United States, but Reed does not put much stock in such statistics despite leading the field.
"I've heard that stat as well, but honestly with how competition is these days, it doesn't really matter where you're from or anything like that," he continued.
"To me you have to be on your game, you have to stick to your game plan. I think the main thing is to stay with my game plan because the odds of me going out and eagling three right out of the gates again, to go and hole out, it's rare.
"So what I can't do is all of a sudden if I go out and throw the first three tomorrow, I'm even par, I can't try to go away from my game plan and get aggressive. I need to stick to my game plan. I know it works. For me, it's more of a mental thing really than anything else."