The giant 39-year-old American had five birdies in his first nine holes as he went out in just 29.

There were no more birdies on the way back but he didn't drop a shot all day and that allowed him to join Jordan Spieth and US Open champion Brooks Koepka in an all-American top of the leaderboard.

Behind them, England's Paul Casey -- who came third in St Andrews in 2010 -- had five birdies and one bogey in a 66 to sit at four-under.

His fellow countrymen Ian Poulter and Richard Bland are three under par along with Justin Thomas and the Canadian Austin Connelly.

Among those lurking at two-under are Japan's world number two Hideki Matsuyama, Sweden's Alex Noren, the world number nine, and twice former winner Ernie Els of South Africa.