Sloane Stephens of the United States celebrates winning her Quarterfinal match against Serena Williams of the United States during day ten of the 2013 Australian Open at Melbourne Park on January 23, 2013 in Melbourne, Australia. (Photo by Mark Kolbe/Getty Images)

In an American versus American upset, rising tennis star Sloane Stephens upset Serena Williams to reach the Australian Open semi-finals.

At just 19 years old, Stephens beat the 15-time Grand Slam singles title holder in three sets.

Towards the end of the second set Williams sustained a back injury, which may have contributed to her defeat. Stephens ultimately won the set, and Williams needed a medical timeout following the set.

Frustrated after three games of the final set, Williams broke her racket by smashing it on the court.

After arguably the biggest win of her career, Stephens told ESPN that, “it’s pretty cool. To be in the semis of a Grand Slam is definitely I say a good accomplishment. A lot of hard work.”

“I’m almost relieved that it’s over because there’s only so much I felt I could do,” Williams expressed in a comment after the game.  “It’s been a little difficult,” she continued.  “I’ve been thrown a lot of (curve) balls these two weeks.”

One of her icons growing up, Williams’ photo is on Stephens’ wall at home.

“This is so crazy. Oh my goodness,” a tearful Stephens said in a post-match interview. “I think I’ll put a poster of myself (up) now.”

Stephens will face No. 1 singles and defending champion Victoria Azarenka in the semi-finals of her first Grand Slam.

Follow Carrie Healey on Twitter @CarrieHeals