WASHINGTON – Juan Martin Del Potro became the first man since Andre Agassi in 1999 to win back-to-back titles at the ATP Washington Classic, rallying to defeat Andy Roddick 3-6, 7-5, 7-6 (8/6) in Sunday's final.

Argentine second seed Del Potro fired the last five of his 19 aces in the tie-breaker and made his first career title defense on his fourth match point when a line call review confirmed that his forehand winner caught the line.

"It was so close," Del Potro said. "The difference in the final points was thin. Unfortunately only one of us can win."

The 20-year-old from withstood hot and muggy conditions to capture his sixth ATP title, his second of the season after Auckland in January and a 300,000-dollar top prize.

US top seed Roddick, in his first event since a five-set Wimbledon final loss to Roger Federer, fired 21 aces but failed to claim his fourth Washington title and 28th career crown at the 1.4 million-dollar hardcourt event.

Sixth-ranked Del Potro won his only prior match with Roddick in last year's Los Angeles final.

Del Potro opened the tie-breaker with his fastest serve, a 137-mph ace, took a 3-1 lead on a second-serve ace and jumped ahead 4-1 on a 136-mph ace. His fourth ace gave him a 6-3 edge and three championship points.

Roddick saved the first with an ace and Del Potro followed by sending a backhand wide and a forehand long, sustaining Roddick's hopes.

But Del Potro fired his final ace and captured the match on the replay call.

Roddick saved a break point in the opening game with a service winner and took 10 of 11 points in a key stretch, including a break in the sixth game, to win the first set.

Del Potro surrendered the break on his second double fault of the game, caught landing just wide on a review challenge.

Roddick and Del Potro traded breaks in the eighth and ninth games of the second set, the South American again succumbing on a double fault, but Roddick netted a forehand in the 12th game to send the match to a third set.

Roddick broke in the third game of the last set but surrendered a break at love with a double fault in the sixth game, setting the stage for the final tie-breaker drama.