The Us Open grandslam will begin from Aug. 29 at Flushing Meadows.
The 18-year-old Mmoh, who trains at IMG Academy in Bradenton, beat 17-year-old Vasil Kirkov, also of Bradenton, 6-1, 6-2, 6-3, in Kalamazoo, Michigan.
The 16-year old Day, from Santa Barbara, California, topped 18-year old Nicole Frenkel of Winchester, Massachusetts 6-2, 3-6, 6-1 in San Diego. Day reached the girls' semifinals at Wimbledon this summer.
On Saturday, 20-year-old Ernesto Escobedo of West Covina, California, wrapped up a spot through the USTA Pro Circuit's US Open Wild Card Challenge.