Matt Cain threw five hitless innings, and Washington was stifled, 3-1, by San Francisco on Sunday.
The win gave the Giants (61-44) a four-game series split with the Nats (61-44).
Cain (3-6, 5.53 ERA, 1.50 WHIP) left after five innings, after striking out five and walking four, on 93 pitches, 48 of them strikes.
Washington got on the board in the sixth on an RBI single by Anthony Rendon (.261/.344/.433, 15 HR, 54 RBI), but that was it for the day for the Nationals.
Gio Gonzalez (6-9, 4.29 ERA, 1.36 WHIP) took the loss, giving up two runs, one earned, on six hits in six innings, striking out one and walking two.
The Nats have a four-game lead over Miami (57-48) in the NL East.