When Kelly stepped to the par-3 fourth hole, he was two shots behind Els, who looked like his smooth-swinging, Big Easy persona the first two rounds. But Kelly stunningly caught his playing partner with a 4-iron shot that hit the green, bounced to the flag stick and dropped in.

And Kelly wasn't done.

He tapped in for birdie on the next hole to move in front, then added two more birdies to extend the lead to two shots. When Kelly knocked in an 8-footer for birdie on the 12th hole, he was three strokes ahead of the field.

Kelly finished with a 67 and, at 13 under, stood a stroke ahead of Els (71), who drained a 35-foot birdie putt on the 18th hole, and Kevin Na (66).

Masters winner Zach Johnson kept up his inspired play after his emotional, draining triumph at Augusta National. His 66 left him four shots back along with Stephen Ames (64), Vaughn Taylor (67), Sean O'Hair (69) and Stephen Leaney (70).

Els seemed ready to end his nearly three-year PGA Tour victory drought and cap a lengthy comeback since knee surgery in 2005. He made only one bogey the first 36 holes, his two 65s leaving him a shot off the tournament's halfway scoring mark.

But Els was off target from the start today. He drove right into some trees on the first hole, then struck a tree with his next shot. His third landed in a bunker and Els ended with a double-bogey 6. He steadied himself two holes later with a birdie that pushed him two shots in front of Kelly -- and set up Kelly's dramatic ace.

After the tee shot went in, Els high-fived Kelly, who raised both arms in celebration. When Kelly pulled the ball from the cup, he gestured to throw it into the gallery before abruptly stopping and shaking his head "No," a big smile on his face.