10 and 11 before a birdie at the 12th and six pars to finish.
An, who has a European Tour title on his resume and three runner-up finishes on the US Tour, said scrambling was the key to his third round - particularly a par save after an errant drive at No. 15.
Poston shot an 8-under 62 on Sunday for a one-stroke victory at the tour's regular-season finale.
An and Poston remain the only players without a bogey on his card this week. Should he win Sunday, he is projected to move from No. 82 in the FedEx Cup standings to No. 27. The gap between Spieth and the top of the leaderboard is only four strokes, as Byeong-Hun An holds a one-stroke edge over Brice Garnett at 13-under-par through two rounds.
He arrived at 18 needing a birdie to force a playoff and was in the trees off the tee.
"Nothing was really working at the end", An said. For a while midway through An's round, six players were tied for the lead at 13 under.
Defending champion Georgia Hall was 4 under after a 2-over 74.
Still, Svensson, who is winless on tour, had his second 61 this year. "I'll be on the Korn Ferry Tour no matter what happens kind of unless I play really well tomorrow, and to be in the spot where I am right now after college, that's a pretty good spot to be in".
He shot a 5-under 65 on the Sedgefield Country Club course in Greensboro, N.C.
He will play the final round with local favourite Simpson, who lives in nearby Charlotte and recorded his first tour victory here in 2011.
"I had a couple missed putts coming down the stretch", he said.
He used a spotless front nine to put pressure on An, including a birdie on the par-4 9th hole. Poston took sole possession of the lead with a birdie on that hole after placing a bunker chip 6 feet from the flagstick. He took a penalty stroke, then landed his third shot left of the green, left his chip 35 feet short and dropped to 20 under after two-putting for his first bogey of the tournament. He made back-to-back bogeys at Nos. He sent his tee shot into the trees and recovered by landing his second shot 60 feet from the pin.
Simpson is in contention in the tournament where he claimed his first title in 2011 and finished second past year.
"I was talking to Danny (Sahl), my caddie, who said, 'You know what? I thought I would be a little more nervous than I was". "So yeah, it was on my mind this week". Nine players started this tournament with a shot at some of that cash, and at the midpoint, two of them - Simpson and Paul Casey - are in position to claim it.
The other annual subplot at Sedgefield focused on the players on the playoff bubble. The FedEx Cup playoffs will begin with The Northern Trust on August 8.
The top 125 qualify for the playoffs and guarantee themselves PGA Tour cards for next season.
"I definitely knew what I was playing for", Kizzire said.