Washington bungled the 10th inning after fighting back from an early 4-0 deficit, and the five-run Reds rally led to a 9-4 Cincinnati win Saturday night.
Down 4-1 in the eighth, Danny Espinosa (.230/.333/.449, 16 HR, 43 RBI) hit a two-run homer, and Bryce Harper (.258/.402/.485, 16 HR, 47 RBI) later added an RBI single to tie the game at 4-4.
But the Nats (49-33) would leave the bases loaded in the eighth, setting the stage for the Reds five-run outburst in the 10th, aided by errors by Harper and Ryan Zimmerman.
Sammy Solis (1-3, 2.17 ERA, 1.03 WHIP) took the loss, charged with four runs, three earned, on three hits in a third of an inning.
The loss ends a six-game Washington winning streak.