Curtis won the Open Championship at Royal St George's in 2003
American Ben Curtis led by one shot after the rain-hit third round of the USPGA Championship at Oakland Hills.
Curtis edged past overnight leader and countryman JB Holmes (70) with a 68 to reach two under par.
Holmes and Henrik Stenson (68) were one under, while Padraig Harrington fired 66 to climb to one over alongside Sergio Garcia (69) and Charlie Wi (71).
Andres Romero and Jeev Milkha Singh are two over, but England's Justin Rose slipped back to four over after a 74.
"Any time you shoot red numbers on this golf course you've got to feel good," said Curtis, who won the Open at Royal St George's in 2003.
"Knowing that I can do it and I've done it before is huge. I've just got to go out there and play my game."
Four hours were lost because of storms on Saturday, with only 25 of the 73 players finishing their rounds, and six - Curtis, Holmes, Rose, Stenson, Wi and David Toms - unable to start.
One player who did complete his third round on Saturday was Argentine Romero, who shot a 65 to leap to two over, and after play resumed on 1215 BST he was joined by India's Singh (70).
Colombia's Camilo Villegas (67), Americans Steve Flesch (70) and Toms (72) and Australian Aaron Baddeley (71) ended three over.
Alongside Rose on four over were Northern Ireland's Graeme McDowell and Thailand's Prayad Marksaeng, who both took 68, world number two Phil Mickelson (71) and Argentina's 2007 US Open champion Angel Cabrera (72).
Harrington was five over after eight when play was suspended at 1915 BST on Saturday and he returned to birdie the 9th and make four more in a row from the 13th before ending with a bogey on 18.
"It feels disappointing bogeying the last," said the Irishman. "I needed to make birdies, full stop. My attitude was make birdies."
The 36-year-old said the course, which had caused so much angst over the first two days, was playing much easier after Saturday's the torrential rain.
"The greens and the fairways are soft. Even though the ball is not travelling as far because it's cold, once you're in position off the tee you can fire at all the pins, they're much more accessible."
South Africa's two-time US Open champion Retief Goosen ended five over after a 69, as did Scot Alastair Forsyth (70), while Ernie Els and Robert Karlsson were one further adrift.
English pair Paul Casey (72) and Ian Poulter (73) finished their third rounds on eight over.
The final round, which will be played in threesomes going off from both the 1st and the 10th, is scheduled to begin at 1720 BST.
The last major champion to play 36 holes on the final day was Seve Ballesteros in the 1979 Open, while Tiger Woods played 27 holes on the Sunday when he won the rain-delayed Masters in 2005.