(CNN) In tennis, it's beyond difficult to transition from teen prospect to grand slam winner. Roger Federer, Serena Williams, Rafael Nadal and Maria Sharapova are the high-profile exceptions rather than the rule.

You don't need to tell that to Americans Taylor Townsend or Kristie Ahn, either, but here they are into the fourth round of a major for the first time many years after being tipped for glory.

Townsend continued her winning ways over Romanians when the qualifier defeated Sorana Cirstea at the US Open on Saturday 7-5, 6-2, charging the net in similar fashion that earned her a stunning victory over Wimbledon champion Simona Halep two days earlier.

"Honestly, my coach and my friends did a good job of just keeping me centered," she told reporters. "I was really glad that I was able to play doubles just to keep me on track and keep me in my rhythm of being out on the court, doing my routines and doing what I have to do, and keeping you in that competitive state of mind and in that competitive mind frame."

Later, wildcard Ahn overcame a leg injury to down the error-strewn 2017 French Open champion Jelena Ostapenko, 6-3, 7-5.

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