The Nationals overcame an early three-run deficit and continued what they hope will be a turnaround campaign in 2010. Washington has not been a game above .500 this late in the season since it had an 81-80 record going into the final day of the campaign in 2005.
Washington rallied for four runs in the third, two of them coming in on Adam Dunn's double.
The score was deadlocked at 4-4 going into the eighth, which Josh Willingham began with an infield single. Willie Harris sacrificed Willingham to second and, after Adam Kennedy was intentionally walked by Rafael Betancourt (0-1), Nieves doubled home the tiebreaking run.
A sacrifice fly by Ivan Rodriguez brought in Kennedy from third.
Washington third baseman Ryan Zimmerman left the game in the seventh inning after pulling up with an apparent hamstring injury. He was recently held out of the lineup for seven days with a hamstring pull.
The win went to Tyler Clippard (3-0), who pitched two scoreless innings. Matt Capps worked the ninth and has earned a save in seven of Washington's eight wins.