While Paul Casey fell back late in the second round of the Tour Championship in Atlanta, playing partner Furyk produced a brilliant best-of-the-day 65 to reach halfway on eight under par.
But Donald, with his short game razor-sharp, matched that aggregate with a second successive 66 and they are a stroke ahead of Australian Geoff Ogilvy.
Casey finished with three bogeys and from joint overnight leader now has five strokes to make up.
Furyk, who at The Celtic Manor Resort next week will collect his seventh Ryder Cup cap, was thrown out of the first of the four play-off events for missing his tee-off time - not in the tournament itself, but the curtain-raising pro-am.
That rule was subsequently changed, but Furyk returned the following week and by winning on Sunday may well pocket the near £6.4million FedEx bonus as well as the £860,000 first prize.
He chipped in on the first hole - playing partner Casey also birdied it - and after turning in 33 to be two behind Donald he overtook him by picking up more shots at the 11th, 14th and 15th with a putter he bought recently for under £20.
But Donald matched the birdie on the long 15th, then made a 30 footer at the next and got out of trouble at the two closing holes.
Casey was left wishing he could have done the same.
He was in no fewer than six bunkers on the last four holes and slipped to fifth place.
Defending champion Phil Mickelson, needing a top-two finish to replace Tiger Woods as World Number One, did not make the move he was hoping for.
Mickelson's 72 sent him down from joint sixth into a tie for 13th on one over and he is now nine strokes adrift.
Justin Rose, disappointed to start with a 74, improved six shots on his return to East Lake, although it moved him only from 22nd to 15th place at two over.
As for overall points leader Matt Kuchar, he is alongside Rose while second-placed Dustin Johnson, the player who denied Casey victory at the BMW Championship in Chicago two weeks ago, is joint 23rd.