CHICAGO -- Alfonso Soriano was able to stand and watch this time.
"I never thought about it, to hit a grand slam," Soriano said. "I just wanted to try put the ball in the air, try to score the run because it was the 13th inning. I made a very good swing."
Reliever Chris Sampson (4-2) gave up a leadoff walk on four pitches to Derrek Lee in the 13th, and Aramis Ramirez followed with a single to put runners on first and third. After an intentional walk to Milton Bradley loaded the bases, Soriano followed with a long homer -- his fourth career slam -- over the center-field wall to end it.
"I think that's the best swing I made in the second half," Soriano said.
Jeff Samardzija (1-1) pitched two scoreless innings in relief, and Lee also homered as the Cubs won their fifth straight and maintained their half-game lead over the St. Louis Cardinals in the NL Central.
Soriano had an embarrassing moment in the 11th, when he hit a slow bouncer to third with one out and a runner on first. He didn't run, even though the ball was fair, and continued to stand in the box after he was thrown out at first and booed by the fans.
"The ball hit me on my foot," he said. "If I run, maybe it is a base hit, but as soon as the ball hit me on my foot, I think it is a foul ball so I don't have a motive to run to first base because it hit me on my foot."
Manager Lou Piniella did not discuss the incident much after the game.
"He thought he had fouled it off his foot, yeah," Piniella said.
Added Soriano: "I got the victory tonight and everybody is happy now."
The Cubs had a great opportunity to win the game in the ninth when Bradley drew a leadoff walk off Astros reliever Jose Valverde. Bradley swiped second base as Soriano struck out, and Valverde intentionally walked pinch-hitter Micah Hoffpauir and Kosuke Fukudome, also drew a pinch-hit walk to load the bases.
Valverde got a break when the Cubs couldn't execute a suicide squeeze. Bradley broke for the plate as pinch-hitter Mike Fontenot attempted a bunt, but Fontenot missed and Bradley was tagged out well before the plate by catcher Ivan Rodriguez. Fontenot later ended the inning by flying out to center to boos.
"I tell you what, our guys stuck with it," Piniella said. "Soriano ended it in dramatic fashion. I tell you what, our young pitching really did a nice job today. ... Look, it was a good win for us, it really was. Well pitched on both sides."
Cubs starter Carlos Zambrano allowed a home run to Carlos Lee on an 0-2 pitch in the second inning, while Astros starter Wandy Rodriguez gave up a homer to Derrek Lee -- also on an 0-2 pitch -- in the fourth that landed on Waveland Ave. It was his 20th of the season, matching his home run total of 2008.
"This was a tough one, but we battled hard and played hard," Astros manager Cecil Cooper said. "We had some chances and just didn't get the big base hit."
With runners on first and third with two outs in the seventh, Zambrano ended the inning by striking out Chris Coste. The always demonstrative Zambrano, yelled as he walked to the dugout with the fans on their feet.
Rodriguez allowed a two-out double to Jeff Baker in the seventh. Cooper decided to intentionally walk eighth hitter Hill, which set up Piniella's decision to stick with Zambrano to bat. Zambrano, who earlier doubled, came to the plate with the fans chanting "Let's Go Z!" but he ended the inning and his day by hitting into a fielder's choice.
Rodriguez allowed just one run on six hits, struck out seven and walked two. Zambrano gave up one run and three hits, and worked around four walks by striking out four.
"Wandy pitched really good," Cooper said. "I didn't think he was particularly sharp, but he battled and kept us in the game. He matched 'Z' pitch for pitch and you can't ask for much more than that."
Astros reliever LaTroy Hawkins was ejected in the eighth inning for arguing balls and strikes.
"Let me see," Hawkins said. "Do I want to tell you the version, the truth that's going to get me in trouble or am I going to be the nice guy?
"Well, I thought it was a strike and I told him, 'Come on, I need that pitch. You don't need any help.' And he said, 'Knock it off.' And I waved him off and he threw me out of the game. Maybe he was having a bad day. I thought he had determined who he wanted to win the game anyway."
- Cubs LHP Ted Lilly, on the DL with left shoulder inflammation, also had his left knee scoped Monday, which is not related to his shoulder injury. According to Piniella, the procedure went well. He is expected to be back in mid August.
- RHP Ryan Dempster is scheduled to start Tuesday against the Astros. He has been on the DL with a broken right toe since July 7.