Joh opened with a 3-and-2 victory over Bruins teammate Sydnee Michaels in the second round and then held off 14-year-old Jaye Marie Green of Boca Raton, Fla., 2 and 1 in the third round on the 6,158-yard, par-73 Erin Hills course.
Green birdied the par-5 10th hole to cut Johs lead to one hole, but Joh won the 11th and 14th to push her advantage to three. Green cut the deficit to two holes by taking the 16th, and Joh ended the match when they halved the 17th with pars.
She was really impressive, Joh said. I think players fourteen years old are the most dangerous because they have no fear.
Green was happy to reach the third round.
I feel like I gained so much, Green said, I cant be mad at myself.
Joh will face Lee Lopez of Whittier, Calif., on Friday.
Lopez beat Lynette Duran of Lakewood, Colo., 3 and 1 in the morning, and edged Aimee Neff of Carmel, Ind., 3 and 2 in the afternoon.
In the other lower-bracket quarterfinal, Tiffany Lua of Rowland Heights, Calif., will face Stacey Kim of Columbus, Ga. Lua advanced with a 20-hole victory over Taiwans Tzu Chi Lin, and Kim beat Michelle Bowles of Davison, Mich., 4 and 3.
In the other quarterfinals, Michelle Shin of Cape Coral, Fla., will face Stephanie Kono of Honolulu, and Lauren Doughtie of Suffolk, Va., will play South Koreas Jennifer Song.
In the third round, Shin beat Angela Oh of Maple Shade, N.J., in 19 holes; Kono routed Alexandra Stewart of Peoria, Ariz., 5 and 4; Doughtie beat Emily Powers of Quincy, Ill., 3 and 2; and Song beat fellow South Korean Ha-Na Jang 4 and 3.