The Reds pounded out 19 hits to emphatically hand Washington its first loss of the season. The Nationals had allowed Miami only one run in a three-game sweep to open the campaign, but Washington starter Dan Haren (0-1) gave up six runs in a four-inning stint.
Frazier and Cozart both hit solo shots in the second inning, Cozart added a three-run blast in the third and Frazier hit a two-run homer during a seven-run seventh.
Xavier Paul contributed a grand slam home run in the big seventh inning and Shin-Soo Choo had a solo affair in the fourth.
Homer Bailey (1-0) gave up two hits over six innings to get the easy win. It was the Reds' third straight victory.
Haren, making his Washington debut, allowed four of the homers.
The Nationals had been the last unbeaten team in the majors.