Young gun Spieth leads Masters
Jordan Spieth (C) of the US reacts after making a birdie putt from a bunker on the fourth green during the final round of the 78th Masters Golf Tournament at Augusta National Golf Club on April 13, 2014 in Augusta, Georgia - by Emmanuel Dunand
The 20-year-old Texan would be the youngest Masters winner and the youngest winner of a major since 1931 if he remains ahead at the finish.
Spieth birdied the second and fourth, where he sunk an outrageous bunker shot and was leading by two strokes. But a poor approach to five pegged him back to six under.
He then birdied the par-three sixth after hitting his tee shot to three feet.
Bubba Watson, the 2012 champion who shared the overnight lead with Spieth at five under, dropped back with a bogey at the third, but birdies at four and six put him just one back of his playing partner.
Next best, two strokes back at four under after six holes, was another American, Matt Kuchar, who briefly led before taking a double bogey with four putts at the fourth.
Still in the hunt at three under were 54-year-old Freddie Couples, who reached the turn in 34, Thomas Bjorn of Denmark, Amerian Rickie Fowler and Swedish rookie Jonas Blixt.