Dunn's homer, his 22nd this season, tied the contest 3-3 with two outs in the bottom of the fourth inning.

Jerry Hairston Jr. added two RBI for the Reds, who escaped a ninth-inning rally against closer Francisco Cordero, who got his 18th save.

The winner was Endison Volquez (11-3), a possible starter in next week's all-star game.

The Nationals, who have lost five of six games, got two RBI from Cristian Guzman.

Losing pitcher Collin Ballester (1-1) yielded five earned runs on four hits in 5 2/3 innings.