Argentine Juan Martin Del Potro beat Viktor Troicki of Serbia 6-3 6-3 to win the ATP event in Washington and claim his fourth successive title.
The second seed rallied from an early break down in the opening set to make it back-to-back hard-court titles and take his winning streak to 19 matches.
'Today I was very, very nervous because I was the favourite to win the tournament,' Del Potro said.
'In a final, if you play your best you can win, for sure, but I think today I played more with my mind than my body.'
The Argentine, who won in Stuttgart, Kitzbuehel and last weekend in Los Angeles, is the first man in ATP Tour history to win his first four finals.
He is also set to rise from his ranking of 19th when the new list is published tomorrow.
Having played eight matches in ten days going into the final, Del Potro looked a little sluggish early on and world number 93 Troicki, who beat top seed Andy Roddick on his way to the final, took advantage.
Del Potro, 19, who struggled all day with his serve, hit one of seven double faults to hand Troicki a 3-1 lead.
After losing the first point of the next game, Del Potro took an angry swipe at the ball and the burst of anger seemed to shake him out of his lethargy.
The 22-year-old Troicki pushed a forehand wide to give Del Potro the break back and the Argentine repeated the feat two games later to move ahead 4-3.
Troicki then called the trainer for treatment on his lower back and when he returned, Del Potro held and then broke again to take the set.
For a moment, it looked as if Troicki would be forced to quit, but he came out for the second set and saved two break points in the second game.
The Serbian did not seem obviously affected by his back problem and games stayed on serve until the eighth when Del Potro snatched the break.
Del Potro's nerve almost failed him when he squandered three match points as he served for the title, but he saved three break points before clinching victory with his fifth ace.
