The 22-year-old Nalbandian, seeded eighth, defeated three-time champion Gustavo Kuerten of Brazil 6-2, 3-6, 6-4, 7-6 (8-6). Earlier, the unseeded Gaudio eliminated Australian Lleyton Hewitt, the No. 12 seed, 6-3, 6-2, 6-2.
Hewitt, a two-time Grand Slam singles champion, had 43 unforced errors to 19 for Gaudio, who broke the Australian seven times.
"His backhand is one of the best single-handed backhands out there," Hewitt said. "I tried hanging in there and I tried a few different things ... But I didn't think I was hitting the balls as cleanly as I have been over the last couple of weeks."
On the women's side, Anastasia Myskina of Russia plays Jennifer Capriati of the United States and Elena Dementieva of Russia meets Paola Suarez of Argentina in the semifinals.