DeRosa went 5-for-5 with four RBI, the Cubs had a season-high 20 hits and Chicago scored seven in the seventh Thursday to beat the Cincinnati Reds 12-4.
DeRosa's five hits -- four singles and a double -- were a career high as the Cubs won for just the fifth time in 15 games this month.
"I've never done it before. It's unbelievable. I look back on it and I've done a lot of good things in my career and I've done a lot of bad things. This happens to be a good thing and it comes at a very good time," DeRosa said after the Cubs moved a half game out of the NL Central lead.
"St. Louis -- a little bit of scoreboard watching -- beat up on Milwaukee today and we swung the bats well," DeRosa added. "We closed the gap, a half-game out after playing some pretty subpar ball for us."
Moved up to second in the order in front of Derrek Lee, DeRosa flourished, while Aramis Ramirez and Jacque Jones contributed four hits each as the Cubs salvaged one win in the three-game series against the Reds.
"I told (manager) Lou (Piniella) after the game I like hitting in front of D. Lee. I'll tell you what, I was getting some fastballs to swing at," DeRosa said.
The Cubs are only two games above .500 but moved within a half game of first-place Milwaukee when the Brewers lost again to the St. Louis, 8-0. The Cardinals now come to Wrigley Field for an important four-game series. St. Louis is just 2½ games behind Milwaukee after sweeping three from the staggering Brewers at Miller Park.
"It's going to be an interesting weekend of baseball. The defending world champions coming in here on a roll. Our job is to slow them down a little," Piniella said.
Jason Marquis (10-7) allowed seven hits and four runs -- all of those coming in the second.
And then the Cubs went to work on the Reds' bullpen in the seventh.
Ramirez and Matt Murton singled against Gary Majewski before Jones hit a liner off reliever Mike Stanton's leg for a single to make it 6-4. After another single by Koyie Hill loaded the bases, Stanton walked pinch-hitter Jason Kendall to force in a run before Theriot hit a two-run double.
DeRosa then greeted reliever Mike Gosling with a two-run single through the box to make it 11-4 and Ramirez followed one out later with his second hit of the inning, a run-scoring double.
The Cubs scored four in the first off Livingston, who surrendered 11 hits and five runs in five innings, but Marquis couldn't hold it.
The Reds tied it with four in the top of the second and could have had more if Ken Griffey Jr. had not been tagged out during a rally-killing mixup on the base paths after he singled off the wall in right center.
"We got a little fortunate there with Griffey rounding that first base bag and getting out. That could have been a really big inning there," Piniella said.
Adam Dunn hit his 32nd homer to lead off the second and the Reds loaded the bases on a walk to Scott Hatteberg, a double by Edwin Encarnacion and a walk to Livingston. Josh Hamilton was hit by a pitch to force in the second run and Jeff Keppinger's infield single made it 4-3 with the bases remaining loaded.
Griffey then hit a ball off the wall to score Livingston. But when Hamilton held up at third and Keppinger had to slow up accordingly, Griffey got caught between first and second and was tagged out easily. Brandon Phillips then grounded out to end the inning.
"With one out, you need to go halfway and if the ball falls in you score easily. (Hamilton) just got a little confused and made a mistake," Reds interim manager Pete Mackanin said.
Hamilton took the blame.
"I kind of got caught in between trying to figure out what to do. I talked to the coaches about it and it's still the one part of my game that doesn't feel quite right," said Hamilton, who didn't play professionally from 2003-05.
The Cubs had four doubles in the first while scoring four times. DeRosa, Ramirez, Jake Fox and Jones all had two-baggers off Livingston.
- Griffey, who had three hits, went to his knees to catch Marquis' sinking liner in the fourth and after he returned to his feet replaced the turf over a big divot he created when he landed.
- The 20 hits were also the most allowed by the Reds this season.
- After days of stifling humidity, the game began with a temperature of just 72 degrees.