Kono, 17, began her tense day by forcing a playoff against Maria Jose Uribe with a birdie on the 18th hole of the 6,363-yard course at Tacoma Country and Golf Club. Kono then defeated the 17-year-old Uribe on the second extra hole.
In the afternoon's quarterfinals, Kono beat 15-year-old stroke-play medalist Kimberly Kim with a birdie on the fifth hole of sudden death.
Kim, who began her week with an impressive 10-under 62 in stroke play, had rallied to defeat 16-year-old Mary Michael Maggio 2 and 1 in a morning match.
Also in the morning, Ayaka Kaneko ousted fellow 17-year-old Mina Harigae 1 up. Harigae won the U.S. Women's Amateur Public Links title last month. Kaneko then defeated Sue Kim in 24 holes to join Kono in the semifinals.
Kristen Park, 14, became the youngest semifinalist by defeating Haley Sanders in 19 holes. Park had an easier time besting Stacey Kim in the morning 2 and 1.
Michelle Shin, 16, also is in the semifinals after easily defeating Mitsuki Katahira 5 and 3, then Sarah Brown 4 and 3.