Later, wildcard Ahn overcame a leg injury to down the error-strewn 2017 French Open champion Jelena Ostapenko, 6-3, 7-5.
Townsend reached the top of the junior world in 2012 but is seeking her first title -- or semifinal -- at the highest level.
Bianca Andreescu, currently a teen phenom herself, is Townsend's next challenger after the Canadian beat last year's Australian Open champion, Caroline Wozniacki, 6-4, 6-4.
Ahn back to US Open after a decadeAhn, meanwhile, played the US Open as a 16-year-old qualifier in 2008, acquitting herself well in a first-round loss to then seventh-ranked Dinara Safina.
But instead of turning pro, as she wanted to, the parents of Ahn -- who was born within walking distance of the US Open -- wanted her to go to college and get an education.