Pinch-hitter Jorge Cantu keyed the rally with a one-out double off Drew Storen (4-2). He slid home under the tag of Nationals catcher Wilson Ramos after Ludwick grounded a single off the glove of diving shortstop Ian Desmond, breaking a 4-4 tie.
Ludwick finished 3-for-4 with a pair of RBI and Brad Hawpe also drove in two with a pair of hits in San Diego's win.
Nats rookie starter Yunesky Maya was touched for four runs on six hits in a 4 2/3-inning effort.
Jayson Werth went 3-for-4 with an RBI for Washington.
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