Volynets, 14, and Eason, 16, are products of Tompkins Tennis International, based in Fremont.
Volynets will face No. 4 Nicole Mossmer of La Jolla in the San Diego area on Friday. Mossmer defeated No. 17 Emma Navarro of Charleston, S.C., 7-6 (5), 6-3.
In the round of 16 of the girls 18s, top-seeded Kayla Day of Santa Barbara, Calif., outplayed No. 9 Michaela Gordon of Saratoga in the Bay Area 7-5, 6-1.
Sofia Kenin, the reigning champion who won the $50,000 Sacramento Challenger last month as an amateur, withdrew before the USTA Nationals began. Kenin, 17, would have been seeded first.
USTA NATIONAL CHAMPIONSHIPS
(Northern California results)
In San Diego
Round of 16Kayla Day (1), Santa Barbara, Calif., def. Michaela Gordon (9), Saratoga, Calif., 6-4, 6-3.
QuarterfinalsKatie Volynets (6), Walnut Creek, Calif., def. Rachel Eason (9), Union City, Calif., 7-5, 6-1.
In Kalamazoo, Mich.
QuarterfinalsTrent Bryde (8), Suwanee, Ga., def. Sam Riffice (4), Boca Raton, Fla./Roseville, Calif., 6-4, 6-3.
SemifinalsJohn McNally and Jeffrey J. Wolf (4), Cincinnati, vs. Anudeep Kodali, Durham, N.C., and Sam Riffice (10), Boca Raton, Fla./Roseville, Calif., late.
QuarterfinalsKeenan Mayo (16), Roseville, Calif., vs. Matthew Tsolakyan, Glendale, Calif., late.
Jenson Brooksby (10), Carmichael, Calif., vs. Trey Hilderbrand (13), San Antonio, late.
QuarterfinalsAndrew Ton, Milpitas, Calif., and Maximilian Wild (2), Murrieta, Calif., def. Andrew Fenty, Washington, D.C., and Sangeet Sridhar (8), Glendale, Ariz., 6-4, 6-2.
In San Antonio
FinalRyan Fishback, Geneva, N.Y., and Billy Suarez (4), Huntington, N.Y., def. Alex Lee, Oak Brook, Ill., and Zachery Lim (1), Fairfield, Calif., 6-3 6-1.