Lahiri, who shot 74-70 on first two days, moved up from his overnight tied-40 to tied-27.
Lahiri had a bunch of birdies -- six in third round after seven in second round -- but on the moving day, Saturday, Lahiri gave away three bogeys, as against three bogeys and a double a day earlier.
On Saturday, Lahiri played very well giving himself a bunch of chances, but failed to convert them. He had better greens in regulation stats as compared to first two days, despite being less accurate off the tee.
From a five-shot deficit, Summerhays now has a three-shot lead at 13-under.
Bubba Watson overcame a heckler on the 18th hole with a birdie for a 68 and was four shots behind along with Justin Thomas (69) and Dufner. Rickie Fowler (72) was five behind and Jordan Spieth shot 71 and he was six shots behind.