The rookie right-hander walked three and fanned four.
Jose Guillen hit two doubles, scored once and drove in two runs; Robert Fick and Daryle Ward hit solo home runs; and Alfonso Soriano and Brendan Harris also had one RBI for the resurgent Nationals (30-34), who moved into a tie for third place with Atlanta in the National League East.
Washington has won nine of its last 11 games.
Ryan Howard had two of the Phillies' three hits.
Philadelphia (33-30), which has lost three of its last four games, dropped to 4-6 in 10 games against the Nationals this season.
Losing pitcher Cole Hamels (1-1) surrendered four runs and six hits in five innings.