Dustin Johnson moved into a one-shot lead at the RBC Heritage after overnight leader Shane Lowry suffered three bogeys in four holes on the back nine to drop back into a share of second.
Johnson had late troubles of his own with back-to-back bogeys on the 16th and 17th holes, but those came after three straight birdies between the 13th and 15th - highlighted by a 47-foot putt on the 14th.
The American signed for a three-under-par 68 to move to 10-under for the tournament.
Lowry had held the overnight lead despite being one of a number of players who had not finished their second round before bad weather struck in South Carolina on Friday.
He finished up a second round of 68 on Saturday morning and seemed to carry over the momentum as he made the turn in his third round at three under par, having made a birdie putt from 34 feet on the ninth.
But bogeys on the 13th, 14th and 16th proved costly as the lead slipped from his grasp.
Poulter's round began badly with a double bogey on the first hole, but he steadily recovered with six birdies taking him to 67.
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