The 15-year-old American shaved off 0.02 seconds from the 10-year-old 400m hurdles championship record held by Ebony Collins, to cross the line first in 55.94 seconds.
Xahria Santiago of Canada set a new personal record of 56.79 seconds to take silver medal, while American Brandee Johnson of the United States took bronze in a new personal best time of 57.47 seconds.
As expected, Kenyan runners Willy Tarbei and Kipyegon Bett dominated the boys 800m final by taking gold and silver. Willy Tarbeiwas took the gold medal in an very impressive time of 1:45.58 beating compatriot Bett by 0.28 seconds. Brazilian youngster Luis Fernando Pires collected bronze medal in 1:48.61.