Klahn, who won the 2010 title at Stanford, will face top-seeded John Isner in the second round.
The 18-year-old Rubin, the Wimbledon boys junior winner this summer, had received a wild card into the main draw
Last Sunday, Rubin, from Rockville Centre, New York, won the USTA Boys' 18s National Championship to earn a main draw wild card into the U.S. Open.
France's Nicolas Mahut also advanced, beating Slovakia's Blaz Kavcic 6-3, 6-2. Germany's Dustin Brown topped Colombia's Alejandro Gonzalez 6-3, 6-2; Dutchman Robin Haase topped France's Benoit Paire 6-4, 2-6, 6-2; and Germany's Benjamin Becker beat Spain's Pablo Carreno Busta 6-2, 2-6, 6-1.