Del Potro will face American John Isner, who moved closer to his second title in as many weeks by blasting 29 aces to despatch Russia's Dmitry Tursunov 6-7(7) 6-3 6-4.
Haas raced to a 4-1 lead in the opening set before rain caused a three-hour 10-minute delay. When play resumed, Del Potro won four straight games to take a 5-4 lead.
Third seed Haas recovered briefly, breaking Del Potro to stay in the set, but the Argentine re-asserted his dominance in the tiebreak, seizing the first four points before winning 7-4.
Del Potro broke the 35-year-old Haas to take a 5-3 lead in the second set and closed out the match on serve.
Isner landed 67 percent of his first serves, had only one double fault, and gave up no break-points in winning his first match against Tursunov in just over two hours.
The eighth-seeded Isner, who won in Atlanta last week and is the United States' top men's tennis player with a world ranking of 20, broke Tursunov to love in the fifth game of the final set to grab a 3-2 lead.
That was all the lanky 28-year-old needed, and he sealed the hardcourt match on an unseasonably cool evening in Washington with an ace on match point.
Isner has not beaten Del Potro, the 2009 U.S. Open champion, in three career meetings.
"He obviously serves very well but he returns exceptionally well," Isner said. "He actually just crushes the ball from the baseline. There's a reason he's (ranked) seven in the world. He's so good."
Isner, however, is riding an eight-match winning streak.
"I've served incredibly well these last two weeks in Atlanta and here and it's carried me. There's no doubt about that," he said. "It's always going to be my shot that's going to keep me in most matches.
"I feel like I'm in a pretty good serving rhythm right now. This court helps my serve. I'm riding high on a lot of confidence now so it's nice."
(Editing by Gene Cherry/Ian Ransom)
