The Swede, who shot 63 in the first round, bogeyed the first before sinking three birdies on the front nine. Stenson sank another birdie on the 10th just as the siren sounded to send the players from the course seeking shelter from lightning. He was on 12 under.
I am scrambling a little bit compared to yesterday, Stenson said. It is always hard to follow up such a good round with another one just as good, but I am happy where I am at.
Playing partner Rory Sabbatini was second and was standing over a birdie putt to cut the lead to five shots when the siren sounded.
Rory is a great player, Stenson said. He likes to attack and it is really tricky when he gets going because he can shoot low numbers.
K.J. Choi of South Korea was 4 under after 12 holes and had a birdie in his sights on 13. He was tied for third with Robert Karlsson of Sweden.
Defending champion and Masters winner Trevor Immelman of South Africa was 1 under through 13 after two birdies and three bogeys.
The second-ranked Sergio Garcia had consecutive birdies on eight and nine to go 2 under through 12.