* Del Potro wins third title in DC
* Isner has only six aces (Adds quotes)
WASHINGTON, Aug 4 (Reuters) - Juan Martin Del Potro of Argentina overcame a shaky opening set to defeat big-serving American John Isner 3-6 6-1 6-2 on Sunday and win the $1.3 million Citi Open, underscoring his status as a major threat at the upcoming U.S. Open.
Del Potro, playing in his first tournament since losing to Novak Djokovic in an epic Wimbledon semi-final a month ago, was at his best when it mattered most, converting four of six break-point chances.
After appearing tired and spraying the ball in the first set, Del Potro played near-flawless tennis the rest of the way to dispatch the towering Isner, who won in Atlanta last week and was seeking his third title of the year.
Del Potro had trouble returning Isner's missile-like serves early in the match but broke the American twice in the second set and in the opening game of the third.
"He broke me very early and I couldn't find my way with either my forehand or backhand," Del Potro said after winning the Washington tour stop for the third time.
"I was lucky in the second set to break his serve early. I was excited to come back in the match. I kept fighting all the time."
The 24-year-old Argentine broke Isner in the seventh game of the final set to take a 5-2 lead and brushed aside three break points in the final game to win the match.
Isner blasted 29 aces in his semi-final victory over Russian Dmitry Tursunov but managed only six against the top-seeded Del Potro, including just one in each of the final two sets.
"The aces weren't going to be as high just because of how far he was standing back," said Isner, at number 20 the highest ranked American. "He was so far back the guy calling the sideline had to get out of the way.
"With his reach, I knew he would get to a lot of balls. I'm sure that was his game plan, get my serve back and go from there. (My lack of aces) was more his doing than anything."
With the U.S. Open beginning later this month, Isner said rated Del Potro just behind the two top-ranked players in the world, Djokovic and Andy Murray.
"I would put him maybe the smallest hair behind guys like Djokovic and Murray," he said. "He could very easily right now be the third favorite.
"He's got a very good shot to go deep (in the tournament)."
Isner said he thought Del Potro, who played the night match on Saturday and didn't get to sleep until 3 a.m., appeared listless in the first set but recovered quickly.
The American said Del Potro "definitely raised his level" of play in the second set. He said the service break to start the third set was crucial.
"I was telling myself whether I lost the set 6-1, like I did, or 7-6, there was still a third set. I was liking the position I was in, especially serving first.
"If I could have gone up 1-0 it might have been a little bit different. But he played a good game. He was tough from that point on.
"It's disappointing but I wasn't as good as my opponent today. It's as simple as that. My hat goes off to him. He was just the better player today." (Editing by Julian Linden)