On a humid afternoon at Wrigley Field, Harris hit his first career grand slam and added a two-run homer as the Nationals rallied from a four-run deficit to rout the Cubs 13-5.
"For me, I just had probably the best game of my career. We got in late and I was kind of tired today, but a lot of guys say when you're tired you have your best days," Harris said.
The Nationals' victory came less than 24 hours after they'd ended a 12-game losing streak in Philadelphia. And they didn't get to their Chicago hotel until 1:45 a.m.
Entering the game with the worst record in the majors, the Nationals were facing the Cubs, the team with the best mark.
"We got beat up today. What are you going to do?" Cubs manager Lou Piniella said. "They came out swinging the bats. They hit the ball all over the park and out of it."
Harris hit his slam off reliever Neal Cotts to cap a six-run sixth inning that put the Nationals ahead 6-4. He added a two-run shot in the ninth off Chad Gaudin for his first multihomer game in the majors.
Harris and Cotts were teammates from 2003-05 with the Chicago White Sox and played on a World Series winner together.
"You know, it's good to hit a grand slam, but I would rather it have been against somebody else," Harris said. "Because Neal and I go way back. ... I consider him a friend. I have to do my job too but if I could choose someone else, I would."
Cotts was part of a Cubs' bullpen that was charged with nine runs, six to Gaudin. Cotts came on after Jason Marquis lost it in the sixth after he'd allowed only two hits in the first five innings.
"I walked the first guy and put us in a bigger jam than we were already in. I put one over the plate and he [Harris] put a good swing on it," Cotts said. "He looks pretty good over there. Today he swung the bat well."
Lefty John Lannan (7-12) allowed seven hits and five runs in 6 1/3 innings to beat the Cubs for the second time this season.
Lastings Milledge hit a sacrifice fly and Belliard had an RBI double to slice the lead to 4-2 and finish Marquis. Pinch-hitter Austin Kearns worked a walk from Cotts (0-2) to reload the bases and Harris hit a drive over the right-field wall for his 11th home run.
Boos came down from the crowd of 40,513 as the Cubs came off the field after the Nationals' big inning.
Harris wasn't through. He fielded Mark DeRosa's liner as it bounced off the left-field wall in the bottom half and made a strong throw to second to nail him.
Boone delivered his big double off Gaudin in the eighth to break it open. Harris and Belliard homered in the ninth.
"I know I'm strong enough to hit the ball out of the park, but I'm not up there trying to hit the ball out of the park," said Harris, whose 12 homers this season are five more than he had in his entire career before 2008. "I don't know where it's coming from and I'm not trying to figure it out."
Chicago's Derrek Lee hit his first homer since July 27, a solo shot and his 18th of the season, to make it 2-0 in the third. DeRosa cleared the left-field bleachers with his 16th homer in the fourth, a two-run drive that gave him a career-high 75 RBIs.
The Cubs scored in the first on a double steal by Lee and Alfonso Soriano.
The start of the game was delayed 62 minutes by rain. ... Harris' six RBIs tie a career high. He also had six against the Cardinals last season while with the Braves.