Harper opened the Washington first by lining a fastball from Nick Pivetta (1-2) to the opposite field for his 11th homer of the season. With a runner on first in the third inning, Harper crushed a changeup 473 feet to dead center.
He has four homers in four games as the leadoff batter. After flying out to deep right-center in the third, Harper flied to deep center in the sixth and to left in the eighth.
Matt Adams added a two-run homer for Washington and made a leaping catch at the wall to rob Maikel Franco of at least extra bases.
The game was delayed 39 minutes by rain in the bottom of the fourth.
Gio Gonzalez (4-2) worked five scoreless innings, allowing two hits with five strikeouts.
Carlos Santana and Maikel Franco hit back-to-back homers in the sixth for the Phillies.
In the shortest of his 33 career starts, Pivetta allowed six runs on five hits and three walks in one inning plus three batters.
Pivetta, who was acquired from the Nationals in the 2015 Jonathan Papelbon trade, is now 0-2 with an 11.81 ERA against the team that drafted him in 2013.
Phillies: Manager Gabe Kapler said RHP Jerad Eickhoff (lat strain) threw as part of his rehabilitation Friday, while RHP Mark Leiter Jr. (forearm strain) will throw live batting practice Saturday. Both have been on the DL since March. … SS J.P. Crawford (right forearm strain) has resumed some defensive work but has not participated in other baseball activities since going on the DL Sunday.
Nationals: 3B Anthony Rendon went 3-for-4 with a double and played seven innings at third base in his second rehab game with Class A Potomac Friday night. Manager Dave Martinez said before the game Rendon will “probably” be activated Saturday. . RHP Shawn Kelley (right elbow irritation) and LHP Matt Grace (left groin strain) threw bullpen sessions Friday. Kelley will make a rehab appearance with Class A Potomac on Sunday and Grace will throw another bullpen this weekend. … Reliever Trevor Gott left the field with the team trainer after allowing three straight hits in the sixth.
Phillies: RHP Vince Velasquez (1-4, 5.70) is 1-2 with a 6.14 ERA in four starts versus the Nationals.
Nationals: RHP Tanner Roark (2-2, 3.55) is 6-6 with a 3.96 ERA in 17 games — 15 starts — against the Phillies.