The game featured teams with the worst record in their respective leagues.
Washington took a 7-1 lead through two innings and spent the rest of the game trying to slow down a Seattle comeback.
The Mariners closed within a run in the eighth and had the tying run on second before Joel Hanrahan worked his way out of trouble.
Jon Rauch came on in the ninth and gave up a two-out single to Adrian Beltre before striking out Richie Sexson to end the game and earn his 14th save. The Nationals had lost six of their previous seven.
Shawn Hill won his first game of the season after losing his first three decisions. He got the victory despite giving up four runs on 12 hits in five innings.
R.A. Dickey (1-2) took the loss after struggling in the second.
Flores then doubled in two runs and Belliard singled in another.