This year, as we celebrate the 50th anniversary of the US Open, we’re counting down the 50 most memorable moments in the history of America’s Grand Slam. Today, we take a look back at No. 32.

A year earlier, Argentina’s towering and charismatic Juan Martin del Potro introduced himself to the US Open with a scintillating run to the quarterfinals, bowing out against Andy Murray in a hotly contested, late-afternoon encounter. In many ways, he was the sensation of that tournament, a player the fans swiftly came to hold in high regard, a competitor deeply respected by his peers and a man entirely comfortable in his own skin.

Back he came to New York in 2009, and now he was even more of a threat to claim the title. More than three months earlier, he had nearly toppled Roger Federer in the semifinals of the French Open, losing in five spectacular sets. At the US Open, del Potro upended Rafael Nadal, 6-2, 6-2, 6-2, in the semifinals, earning the right to confront the mighty Federer in the title-round contest.

Federer was riding on the wave of a 27-match US Open winning streak. He had not lost since a 2003 setback against David Nalbandian in the round of 16. He was going for his sixth title in a row, a feat that had not been realized at the U.S. Championships since Bill Tilden did it in the 1920s. Federer forged ahead by a set and 5-4 against del Potro and was serving for a two-sets-to-love lead. He reached 30-0, but somehow the rangy Argentine broke him there.

play video 50 Moments That Mattered: Del Potro edges Federer for 2009 title

Del Potro fought back to take the set, dropped the third and then was two points away from defeat when serving at 4-5, 15-30 in the fourth set. But in this clutch corner, del Potro was not found wanting. He unleashed a thundering 128-mph first serve to the Federer backhand, and the top seed sent it back long. Del Potro followed with an ace down the T for 40-30 and held on for 5-5. They went to a tiebreak, and del Potro never lost a point on serve, ruling 7-4 in that sequence.

All across the fifth set, with a significant chunk of the crowd cheering him on vociferously, del Potro blasted Federer off the court with a succession of unimaginably explosive forehand winners. That signature shot carried him to an exhilarating 3-6, 7-6, 4-6, 7-6, 6-2 triumph.

Del Potro established himself as only the second man from his nation to secure the US Open singles title. It was an immensely popular triumph for the No. 6 seed. This was the seminal moment of his career.

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