By Steve Ginsburg

WASHINGTON (Reuters) - Juan Martin Del Potro spoiled Andy Roddick's return to the tour after a month-long hiatus by upsetting the top seed and world number five 3-6 7-5 7-6 to win the Washington Classic on Sunday.

The 20-year-old Argentine won the final-set tiebreak 8-6, clinching the 2- hour slugfest with a scorching forehand that was ruled good after an appeal.

"It was so difficult," Del Potro said after successfully defending his title. "When you are so close in the third set, you don't want to lose. I was fighting with everything I had."

Roddick, playing his first tournament since losing to Roger Federer in the Wimbledon final last month, saved a match point on Del Potro's serve at 5-6 in the tiebreaker.

Del Potro, however, aced his next serve and then sealed his second tournament win of the year with a forehand that Roddick challenged, but which the replay showed had just nicked the line.

"I thought it might have been out," said Roddick. "I asked him when he was standing there (at the net) and he also said he thought it might have been out.

"Imagine his disappointment when it wasn't," he added with a smirk.

With temperatures hovering around 35 degrees Celsius, Roddick said he believed the second seeded Del Potro was trying to end points quickly to save his energy.

"Today was brutal," said Roddick. "It was hot. I think he might have been feeling it more than I did. He didn't want to get in the long points and was going for broke there at the end.

"He was rolling the dice. I forced him to play high-risk tennis with the conditions. He was taking big cuts, especially for the last 30 minutes, and connecting."

Del Potro gift-wrapped the first break of the final set when he punched an easy overhead wide and then double-faulted on consecutive points to give Roddick a 2-1 lead.

Roddick handed the donation back, however, losing the sixth game to love by double-faulting at break point.

"I played one bad point and that was the double (fault) on break point," said Roddick. "To get to love-40, I put in three first serves. He was taking big swings and converting."

The 26-year-old Roddick, who spent the last month recovering from a hip injury, said he was pleased with his effort, especially with the U.S. Open on the horizon.

"I'm not far off from where I was toward the end of Wimbledon," he said. "Coming in after being off for a month you want to fine out where your form is at.

"To come in here and beat some players and have a look at winning this week, I feel a lot better about my game now than I did when I first got here."

(Editing by Nick Mulvenney)