A Lorenzo Cain walkoff single capped a three-run Kansas City rally in a 7-6 Royals victory over the Nats on Tuesday.
The world champs touched up Washington closer Jonathan Papelbon (0-2, 4.50 ERA) for three runs on five hits in the ninth.
Nationals starter Tanner Roark had been on the hook for the win after giving up four runs, three earned, on six hits in seven and a third innings.
Papelbon was charged with his second blown save of the season.
KC entered the ninth trailing 6-4, but after getting the first two men on, the Royals (14-12) tied the game on a two-run single by Mike Moustakas.
After Jarrod Dyson lined out to left for the second out of the inning, Alcides Escobar singled to put runners on the corners.
Cain stroked an 0-2 pitch from Papelbon to left-center to plate Moustakas with the winning run.
Anthony Rendon, Daniel Murphy and Jayson Werth all homered for the Nats (18-8), who maintain a game and a half lead over the New York Mets in the National League East.