The 24-year-old top seed from Argentina shook off a sluggish start to beat the US eighth seed 3-6, 6-1, 6-2 for his third title in six years at the $1.76 million ATP and WTA hardcourt event, a tuneup for the year's final Grand Slam event beginning in New York in three weeks.

World number one Novak Djokovic, defending champion Andy Murray, five-time US Open winner Roger Federer and 2010 US Open champion Rafael Nadal were del Potro's picks as the players to beat at Flushing Meadows.

In his first event since an epic five-set loss to Djokovic in the longest semi-final in Wimbledon history, del Potro showed his left knee was fine after an injury at Wimbledon.

Del Potro stretched his Washington win streak to 14 matches by adding to the titles he won in the US capital in 2008 and 2009. The last time he won in Washington he went on to win the US Open, the only Grand Slam title since the 2005 Australian Open not taken by Federer, Nadal, Djokovic or Murray.

Seventh-ranked del Potro won his 15th career ATP title, his second of the year after Rotterdam in February, and improved to 4-0 all-time against Isner, the ATP season aces leader, despite not getting to sleep until 3 am after a rain-interrupted semi-final.

Isner, ranked 20th, missed out on his third crown of the year after title runs last week in Atlanta and in April at Houston.

Del Potro broke Isner in the fourth and sixth games of the second set, in each taking an Isner overhead smash and firing back a lightning-like forehand winner.

Isner netted a forehand to surrender a break in the seventh game of the third set and del Potro saved three break points in the final game before taking the title after 89 minutes on a backhand cross-court winner.

Slovakian seventh seed Magdalena Rybarikova won her second Washington title in a row, beating German Andrea Petkovic 6-4, 7-6 (7/2) in the women's final, for her fourth career WTA crown and her first of the year. (AFP)