Duncan, the 22-year-old from Shreveport, La., who recently completed her college career at LSU, was a stroke behind Curtis Cup teammate Courtney Swaim after the first of two rounds of stroke-play qualifying on the Sleepy Hollow Country Club course.
The low 64 scorers after the second round will advance to match play, beginning Wednesday. The 36-hole final is Saturday.
Duncan, who clinched the United States' Curtis Cup victory over Britain and Ireland two weeks ago in Pittsburgh, is trying to reclaim the Robert Cox Cup after shipping the trophy back to the U.S. Golf Association four weeks ago.
'If that wasn't motivation to repeat, then I don't know what is,'' said Duncan, who will turn pro after the tournament.
Duncan revealed her hiding place for the Cox Cup - the bathtub in her family's home.
'When I was in college, we had a security expert speak to my sorority,'' Duncan explained 'He said that's one of the best places to hide something in case there's a robbery. What are they going to steal in the bathroom - shampoo or a razor?
'So when I'm traveling or when no one is at home, the trophy went into the tub, and I pulled the shower curtain to hide it.''
Swaim, the Sugar Hill, Ga., player coming off her final year at Auburn, had six birdies and two bogeys on the hilly course.
'I clubbed myself well today, both uphill and downhill,'' she said. 'I've been playing well the last three weeks, and I gave myself good chances today by hitting it close.''
Swaim also is making her final amateur appearance.
'You don't come here to play unless you want to win,'' she said. 'I can't wait to get this stroke play over with. Hopefully, I'll be here Saturday.''
Maru Martinez of Venezuela, playing her first USGA event as she prepares for her freshman year at Auburn, and Danielle Downey of Spencerport, N.Y., opened with 69s, and 16-year-old South Korean star Aree Wongluekiet topped a group at 70.
Wongluekiet was 5 under after nine holes, but finished with a 4-over 39 on the front nine.
Wongluekiet tied for 10th in the LPGA Tour's 2000 Nabisco Championship, and also tied Angela Jerman as the low amateur this year in the U.S. Women's Open at Prairie Dunes. She won the 1999 U.S. Girls' Junior, becoming the youngest champion in tournament history at 13 years, 3 months, 7 days.
Canada's Alena Sharp also shot a 70 that included the first hole in one of her career. Sharp, a senior at New Mexico, used a 6-iron to ace the 194-yard seventh hole.
Carol Semple Thompson, the 53-year-old star coming off a strong performance in her 12th Curtis Cup appearance, shot a 72. Semple Thompson, from Sewickley, Pa., won the 1973 tournament for the first of her six USGA titles. She has won the last three U.S. Senior Women's Amateur titles.
Duncan, with father Dave serving as her caddie for the second straight year, had four birdies and a bogey.
'I'm still on a high from the Curtis Cup,'' she said. 'Even though I didn't play my best golf, I've worked on my game since then and I expect to do well here.
'There is no particular number in my head. The last thing I want to do is put extra pressure on myself. That's when you don't accomplish anything.''