Del Potro, a two-time titlist at the Washington event, waited out a 3-hour rain delay to overcome Germany's third-seeded Haas 7-6 (7-4), 6-3.
The 24-year-old Argentine reeled off his 13th consecutive win in at the Citi Open, which he claimed in 2008 and 2009.
Isner, meanwhile, continued the momentum he generated with last week's victory in Atlanta, scoring a 6-7 (7-9), 6-3, 6-4 triumph over Russian Dmitry Tursunov in a match held up by a 1-hour rain delay.
The 6-foot-9 Isner fired six consecutive aces to end the match en route to his 15th career final.
Notable deaths of 2014 [PHOTOS]