Jason Dufner took over at the top of the RBC Heritage leaderboard as Graeme McDowell and Shane Lowry both carded third round scores of 69 further down the field at Harbour Town Golf Links.
The American surged into first place with a brilliant 65 as the former US PGA Championship winner twice made eagles on the front nine - from 14 feet on the second and a hole-out from the fairway on the fifth - and although there were three bogeys on his card they were offset by five birdies.
That left Dufner 13 under par, one ahead of DeLaet, who signed for a two under 69.
McDowell shot a two-under 69 to move to four under for the tournament, tied for 31st, while compatriot Shane Lowry, who at one stage got to six under, is one shot further back.
Lowry also signed for a 69 thanks to three dropped shots in his final four holes
"It was a little bit of a roller coaster but it was good to pick up a couple of eagles," Dufner told the PGA Tour website after his blistering round.
"The back nine was really clean, I was really happy with how I played on the back nine - hopefully that will bode well for tomorrow's final round."
Former US Open champion Webb Simpson and American Kevin Kisner are a shot further back in third, with Donald's former Ryder Cup team-mate Ian Poulter fifth on 10 under.
Poulter had put himself in contention with four birdies on the front nine but dropped three shots over the next two holes, including a double-bogey on the 10th after his tee shot found water, before a birdie from 40 feet on the 14th completed his scoring.
