Washington snapped a four-game losing streak with the help of a four-hit performance over seven innings by Dan Haren.
Haren (7-11) retired the first 10 batters he faced before giving up a double to Michael Young and a single to Chase Utley that produced the first Philadelphia run. Haren struck out seven without walking a batter.
Rendon hit a two-run homer and Zimmerman had a solo shot in the second and Washington added a run in the third on an RBI base hit by Adam LaRoche.
The Nationals broke things open with a four-run fifth, during which Haren drew a bases-loaded walk and Denard Span produced a two-run single.
Washington roughed up John Lannan (3-5) for eight runs on nine hits in five innings.