Tennis is nothing if not an unpredictable sport. Surprising upsets are as much a part of the game as storied rivalries. Yet when a player seemingly comes out of nowhere to produce a big result, there are those who perennially throw up their hands and wonder “how could this happen?”

As world No. 296 Victoria Duval closed out former US Open champion Samantha Stosur over three grueling sets, the tennis cognoscenti seemed to collectively scratch its head. With few data points available to hint that the young American was capable of pulling off such an upset, many chalked it up to the lights of New York City, or even the higher-ranked player’s own enigmatic history. With nearly sixty unforced errors, the top Australian certainly helped things along as the match edged towards its dramatic conclusion.

In pop culture, many a coming-of-age story is built around “that summer that changed everything.” Having followed Duval’s progress in the months leading up to last night’s massive win, it was clear that there was something cinematic about how she built up to taking down a major champion.

The American teenager got a call after Wimbledon to play for the Philadelphia Freedoms, Billie Jean King’s signature Mylan World TeamTennis team, after rising star Sloane Stephens withdrew with injury.

“Kathy Rinaldi [USTA National Coach] told me that I was invited to come here…it’s been an awesome experience so far,” Duval had said on July 20, ahead of a decisive win over Italy’s Maria Elena Camerin, where she made the Italian veteran look like a club player over the course of thirty minutes.

“I remember seeing Venus and Serena play [Mylan WTT]…I thought it was such a cool concept.”

More invaluable than the experience of playing competitive matches every day was the opportunity to interact with Mylan WTT’s co-founder, Billie Jean King. That weekend, King visited the Freedoms at their home court at Villanova University. Suddenly, Duval was taken under the wing of “The King.

“Every day, I’m getting better,” claimed Duval at the time. “Billie’s helping me with my technique. I think she’s an incredible human being, not only for how much knowledge she has, but just for everything she’s done: not only for women, but for tennis…just to have someone who knows so much, the information is that much clearer!”

Previously known for being a more defensive counterpuncher, Duval began taking more chances from the baseline, setting up points with sharp inside-out forehands and knocking away winners with her backhand down the line.

King could not believe Duval’s progress in such a short period of time.

“We got lucky…Vicky has really improved,” said the legendary champion. “She’s getting more racquet speed on her shots…in two and a half, three days, I would say she’s 100% better. We’re giving her great practice and she gets it. She’s very smart, she grasps things very quickly and she’s producing already.”

Though her TeamTennis season was cut short with an injury, Duval took what she had learned and brought it to the qualifying rounds of the US Open. Opening against No. 4 qualifying seed Teliana Pereira, Duval played with ruthless aggression and compensated for a slow serve with all-court play and powerful groundstrokes.

Winning all three qualifying matches while losing no more than three games in a set, Duval came into the match with Stosur high on confidence and comfort with her surroundings at the aptly named Billie Jean King National Tennis Center.

“Qualifying for a Grand Slam is not an easy task. I played amazing all three of my matches. That definitely helped me for today,” Duval said after last night during her post-match press conference.

Late in the final set, Duval consistently kept the ball deep to the Aussie’s weaker backhand side, producing many of the unforced errors that littered the 2011 champion’s stat sheet. After failing to convert on her first three match points, she took a page out of the Billie Jean King playbook, stepped into the court, and put away one last inside-out forehand to clinch the win. She stands in a section of the US Open draw devoid of big names, former top-five player Daniela Hantuchova the most recognizable among them.

That development might be the most compelling thing about Duval’s career-changing summer. For all that she has accomplished thus far, the season is far from over and the new American star has the chance to build on this win and become the breakout story of the summer.