The 141st-ranked Ahn, the lowest ranked player remaining, has already beaten two former Grand Slam champions to get this far, including Latvia's Jelena Ostapenko 6-3, 7-5 on Saturday. Ostapenko has a French Open title on her resume. She beat Russian Anna Kalinskaya, a former US Open champion, in the second round.

"I lost in a tight set and then she went on to become No. 20 in the world," Ahn said. "It will come down to who takes in the moment better."

Stanford grad and three-time All-American Kristie Ahn plays No. 25 Elise Mertens of Belgium in the fourth round of the US Open on Monday morning at the Billie Jean King Tennis Center in Flushing Meadows, within walking distance of where Ahn was born.

"I think about how awesome an opportunity it is and what a great experience it's been this week," Ahn said. "I feel like its a culmination, a build up of everything and a lot of bad things happened too."

How much longer Ahn plays on playing professionally is up for debate. At 27, she's at a crossroads. Mertens is a rising star; Ahn is in the midst of a dream tournament, a career highlight. No matter what happens this week, can Ahn ever expect to duplicate the feat?

The 23-year-old Mertens, ranked as high as No. 12, has won five WTA titles and 11 ITF titles. Ahn has yet to make a semifinal on the WTA Tour and has won seven ITF titles.

Mertens has reached at least the fourth round of all four Grand Slam events, with a semifinal finish at the Australian Open. She's appearing in her second consecutive fourth round match at the US Open. She also reached the fourth round at Wimbledon earlier.

"You guys have no idea. That was so cool," Ahn said. "I've never played in a stadium that full before. It was pretty awesome to have a majority of everyone cheering for me. It was such an awesome environment."

But Ahn, who helped the Cardinal win the 2011 national title, has found the strength, the motivation to carry on.

This is the stuff of a Cinderella story waiting to happen. There's even a family element to it. In an article published in the New York Times, her parents are quoted as wondering why she hasn't set aside her tennis dreams in favor of job security in the corporate world. They thought she was ready to put down her racket two years ago.

Ahn received a wild card into the US Open main draw thanks to earning enough points in a challenger series. A quarterfinal finish at the Mubadala Silicon Valley Classic in San Jose clinched it. She became the first Challenger (in its eight-year history) to qualify for the Round of 16.

"Lele texted me after my second round win and it was like getting a text from the president," Ahn said. "She taught me how to keep my composure and she does that better than anyone."

She's been working at tennis for five years and has discovered the reasons why she was working so hard at it.

Stanford grad Ahn is on top of her tennis game at US Open