Stenson was among the late starters who had to endure a rainfall that made for higher scoring in the afternoon, but the Swede still managed seven birdies and an eagle to go with a lone bogey.
Gary Boyd of England was second after setting the early clubhouse lead with a 65, despite a bogey on the last hole. Three players were tied for third, a further stroke back.
Stenson eagled the par-5 sixth hole to reach the turn in 5 under. He nearly eagled the 18th as well but missed a 4-foot putt and had to settle for a birdie.
World No. 3 Martin Kaymer was six shots back after a 70.