Second seed Alexandr Dolgopolov survived a three-hour rain interruption to claim his second career title with a 6-7 (7/9), 6-4, 6-1 win over Germany's Tommy Haas at the ATP hardcourt event.
Dolgopolov, ranked 25th in the world, denied the 34-year-old Haas a ninth career title in the US.
"I didn't know quite how to play him before I came out but it worked well and I'm happy I won," said Dolgopolov. "I needed to show my best game.
"I had to attack and make winners against Tommy. It was a tight match, especially in the opening set with the weather coming in.
"I had to play well in the important moments and ignore the conditions," added the Ukrainian who hammered 11 aces.
Dolgopolov needed two-and-a-quarter hours on court to secure his victory in the tough conditions.
Haas struck nine aces but also produced 11 double-faults -- his last as he dropped to 5-1 down in the final set.
Haas, ranked 36th in the world, was bidding for his second trophy this season after beating Roger Federer for the Halle title on grass two months ago.
The German was playing Washington for the seventh time. An agitated Haas complained bitterly in the final set about balls taking repeated dead bounces on a hardcourt which had surface patches where water had soaked through.
The Ukrainian had reached the title match without losing a set before Haas won the first in a tiebreaker -- a set which was interrupted as the German led 6-5.
The final was the fourth for Dolgopolov, who did not play the London Olympics due to a disagreement with his national federation.
Haas accepted defeat without mentioning the court conditions. The German was playing his second final in three weeks after losing to Juan Monaco in Hamburg last month.
"I've yet to win this tournament and there are not a lot of years left for me," said veteran Haas, the former number two in the world.
"I still hope that I will be back here next year to try for it again. It was really rough conditions, waiting out the rain and then playing against a tough opponent."