Washington was able to win the game without third baseman Ryan Zimmerman, who left the contest in the seventh inning because of a right hamstring cramp.
The game was tied at 4 when the Nationals took the lead off reliever Rafael Betancourt. With runners on first and second, Nieves, who started behind the plate because Ivan Rodriguez was given the day off, took an 0-1 pitch and doubled to left-center field to score Josh Willingham.
"He just hung a slider up and away," Nieves said. "I put a good swing on it and put it in the gap. I saw [center fielder] Dexter Fowler out there and he can run, but when I saw it drop, it was a huge double."
Rodriguez entered the game as a pinch-hitter for Tyler Clippard and hit a sacrifice fly to score Adam Kennedy and give Washington a two-run lead.
"Both catchers have been outstanding," Nationals manager Jim Riggleman said. "Great at-bat by both guys. Pudge hit a really tough pitch -- down and away -- and got us an insurance run. Wil did a great job behind the plate. Wil is a pro. He handles his role as the backup catcher tremendously and he gets some big hits for us. We are fortunate in our catching situation. We are pretty good right there."
Clippard (3-0) won his third game of the season, while Matt Capps picked up his seventh save.
Clippard entered the game in the seventh inning and dominated. He pitched two shutout innings and struck out three batters in the process. Clippard has become a quality setup man for the Nationals. He has allowed one earned run in 11 2/3 innings and struck out 14 batters, which leads the team.
"I have a good feel for my pitches right now," Clippard said. "My whole career, I have always been successful against lefties and righties. For whatever reason, I have a good changeup and I will throw some pitches that will run in on your hands sometimes."
The Nats are 8-7 to start the young season. A year ago, Washington was 3-10. What's the difference this year? Great chemistry and better pitching, according to Nieves.
"We have a real good chemistry this year," Nieves said. "We have a nice group of guys and we are having fun. We can hit. We hit last year. We just needed the starting pitchers to keep us in the game. We are going to score a lot of runs and win a lot of games."
Left-hander John Lannan started for Washington, and he kept the team in the game. However, he continued to have problems against left-handed hitters. They went 7-for-9. Carlos Gonzalez had four of the hits, while Brad Hawpe had two hits and Jason Giambi collected a knock.
Lannan threw 107 pitches, but 34 of them came in the first inning as the Rockies scored two runs. Giambi hit a sacrifice fly, while Fowler scored on a groundout by Troy Tulowitzki.
Colorado loaded the bases later in the inning, but Clint Barmes flied out to center fielder Nyjer Morgan to end the threat.
"They had a 10-run first inning against them Sunday, and we had scored 10 runs last night through the first three innings of the game," Rockies manager Jim Tracy said. "If we're able to open it up a little more in the first inning and able to force some more pitches out of John Lannan, you shorten his start and you have the opportunity to get even further into their bullpen."
Colorado made it a three-run game in the top of the third as Hawpe hit a solo home run.
Asked why he is having a tough time getting left-handers out, Lannan said his offspeed pitches are not working. If the pitches are not working, the left-handed hitters are not going to have problems getting on base.
"I felt great in the 'pen," Lannan said. "Everything went away after I got in the game. If my offspeed stuff is not working, I don't have that much to battle with them. I made OK pitches today. I didn't baffle the left-handers. It's something I work on all the time, but it's getting there."
The Nationals took a 4-3 lead in the bottom of the third inning off right-hander Jason Hammel. Adam Dunn highlighted the scoring with a two-run double.
After they tied the score in the fourth inning on a Gonzalez single, the Rockies were blanked the rest of the way.
"You have to do the little things right, and hopefully some calls go our way," Lannan said. "You get base hits here and the relievers and starters go out and put some zeros on the board -- that's all we could ask for."
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.