Within 48 hours Argentina’s Juan Martin del Potro, an name largely unknown to those who only watch Grand Slam Finals, defeated the sports two biggest stars in Nadal and Federer to win the US Open. The 20-year-old’s seventh match against Federer looked like it was headed the same way as the first six – with a loss. Yet, it was Federer who got rattled — arguing with the chair over a replay that went against him — while Del Potro soaked in the moment even high-fiving fans after winning a point.

This win was a statement to the top two players in the world. It not just that he handed Nadal his worst loss at a grand slam event or that he rattled the unflappable Federer, it’s that he’s 20 years old and had been labeled an underachiever for most of his career. At 6-6 del Potro’s height advantage and crushing forehands are well suited for hard court surfaces. So don’t expect Juan Martin to be a one-hit wonder.

Is this the end of Federer? Certainly not. But maybe the way Federer once dominated this sport is beginning to slip away. What else does Roger have to prove? He’s won more Grand Slam titles than anyone in the history of the sport.

For del Potro now comes the realization that you are the hunted. There’s no sneaking up on the big boys anymore. They will not overlook you ever again. How he responds to this new reality will determine whether Tennis has a new star.