In the final, Juan Martín del Potro defeated 5-time defending champion Roger Federer 3–6, 7–6(7–5), 4–6, 7–6(7–4), 6–2 after 4 hours and 6 minutes to win the match.

The 2009 US Open final was Del Potro's first win over Federer, having lost his 6 previous encounters against the Swiss.

Del Potro won the pre-match coin toss and chose to receive, Federer elected to stay.

The first break of serve in this set came in the seventh game, when at 30-all, Federer hit to forehand errors to give Del Potro a 4–3 lead.

In the eighth game Federer broke back, converting his 3rd consecutive break point with a backhand forced error from Del Potro.

Federer faced a break point in the ninth game and responded with 3 unreturned serves to hold for a 5–4 lead.

In the eighth game, Del Potro sent a forehand to the net to give the break back to Federer and level the set to 4–4.

In the eleventh game Federer was 40–0 up but eventually needed to save 2 break points and held serve for 6–5.

A plus-1 inside-in forehand error from Federer gave Del Potro 2 break points in the second game of the set.

Del Potro converted the second break point with a running cross-court forehand passing shot winner to take a 2–0 lead.

Del Potro ended Federer's US Open streak and claimed his first (and only) Grand Slam title.