Lopez, a former shortstop for Cincinnati, tied a career-high with six RBI. He also scored a pair of runs for Washington, which ended a nine-game road losing streak.
Jesus Colome (4-0) got the win after pitching two scoreless innings.
David Ross added a two-run homer for the Reds, who have lost 15 of their last 20 games.
Kirk Saarloss, making his first start this season, gave up three runs and six hits in 5 1/3 innings.
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