Zimmerman went 2 for 4 with two RBI. His run-scoring single in the fifth gave the Nationals the lead for good at 2-1.
Zimmerman finished the series with eight RBI in three games.
Ross (5-5) won his second straight start, allowing the bunt single by Corey Spangenberg and one run in six innings. The right-hander struck out seven and walked two.
The Padres finished with only three hits.
Spangenberg led off the fourth with the bunt single in front of the plate. He ended up at third on the play when catcher Wilson Ramos made a wild throw.
Yangervis Solarte followed with a sacrifice fly that gave San Diego a 1-0 lead.
The Nationals took the lead with two runs in the fifth. Cashner hit Yunel Escobar on the wrist with the bases loaded -- the third baseman left the game an inning later -- and Zimmerman added his tie-breaking single one batter later.
San Diego missed on a good scoring chance against Casey Janssen in the seventh after Melvin Upton, Jr.'s two-out triple. Washington manager Matt Williams then moved some players around after center fielder Michael A. Taylor got hurt on the play and needed to leave, and Anthony Rendon went to third.
Rendon made a diving stop on pinch-hitter Jedd Gyorko's shot down the line and threw him out to prevent a run and keep Washington's lead at 3-1.
Nationals: OF Denard Span (back soreness) was given a day off to rest. Williams said Span is not dealing with the back spasms that put him on the DL, and this soreness is something he will deal with the rest of the season. . Escobara left in the sixth, an inning after getting hit on the right wrist with the bases loaded. He was down for a short period of time and finished the inning before leaving. . Taylor came out in the seventh after crashing into the wall -- and tearing a piece of the padding -- trying to make a leaping catch of Upton's triple. Taylor appeared to have a right knee injury.
Padres: OF Matt Kemp is out of the lineup due to a sore left shoulder that he hurt diving for a ball in Wednesday's game. He's day-to-day. . RHP Josh Johnson (Tommy John surgery) threw off the mound and felt good Wednesday. He will throw again in a few days.
Padres RHP Ian Kennedy (8-11, 4.01) starts the opener of San Diego's weekend series in Philadelphia. The Nationals send RHP Max Scherzer (11-10, 2.79) out to kick off their series with Miami on Friday.