The 22-year-old Knost, who made the cut at the PGA Tour's EDS Byron Nelson Championship in April, will face Cody Paladino in Saturday's 36-hole final. The 18-year-old from Kensington, Conn., beat Derek Fathauer of Jensen Beach, Fla. 1-up.
Knost came back from a two-hole deficit against Nagy, an 18-year-old from Charlotte, N.C., with a string of birdies on the back nine.
Paladino outlasted Fathauer, who had played 81 holes in two days.
Knost advanced to the semifinals with a 2-up victory over Robert Rohanna of Waynesburg, Pa., and, along with Paladino, earned an automatic exemption into the U.S. Amateur in August.
Nagy advanced to the semifinals after ending Andy Roberts' improbable run to the final eight Friday morning. Nagy defeated the 64th-seeded accountant from Owensboro, Ky., 1-up.
In the quarterfinals, Fathauer beat Daniel Im of Fullerton, Calif., 1-up in 18 holes after playing 45 holes on Thursday.
Paladino reached the semis by defeating UCLA sophomore and Brazilian Lucas Lee 1-up in the quarterfinals.
The winner of the final will likely receive an invitation into the 2008 Masters Tournament should he remain an amateur.
Copyright 2007 Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.