Although Sean Halton's first career grand slam was the big blow, it was Chris Rusin's pitches out of the strike zone that proved more costly to the Cubs.
Halton's homer capped a five-run first inning that included two walks by Rusin (2-5) and Tyler Thornburg allowed just two singles over six innings as the Milwaukee Brewers beat Chicago 7-0 on Wednesday night.
"Obviously he got the ball up a couple times in bad situations, but I think the walks were the biggest thing that happened," Cubs manager Dale Sveum said. "That first inning, he had a chance of getting out of there and walked a guy and then Halton gets a 3-2 pitch up in his eyes and hits it out of the ballpark."
Jean Segura walked with one out in the first and advanced to second on Jonathan Lucroy's single. Aramis Ramirez singled in Segura. After Carlos Gomez flied out, Khris Davis walked to load the bases. Halton then drove a 3-2 pitch 370 feet over the right-field wall for his fourth home run.
"When I needed to make a pitch, I left it up and they hit it out of the yard," said Rusin, who has not won in seven starts since his last victory on Aug. 9. "Those are the games you've just got to learn from. Just keep the ball down is what I keep telling myself. But that first inning really hurt the team. Just took the life out of us. As a starting pitcher, you don't want to do that to a club."
Thornburg (3-1) struck out four and walked two as he reached six innings for the sixth straight start since being recalled from Triple A Nashville on June 29. He has allowed five earned runs in 36 innings as a starter and has also made 11 relief appearances for the Brewers this season, posting a 1.96 ERA. At Nashville, he was 0-9 with a 5.79 ERA in 15 starts.
"I was a little upset being in Triple-A," said Thornburg, who made three starts in eight appearances last season with the Brewers. "Honestly I was kicking myself a little bit too much. Doing things I wouldn't necessarily do. Up here I stuck with my three good pitches and I am really trying to improve those."
Jimmy Nelson pitched two innings and Michael Blazek got the last three outs to preserve the four-hit shutout.
Davis led off the fourth inning with his 10th home run to put the Brewers up 6-0.
"Today, I hung a change-up and Davis hit it out," Rusin said. "High fastball and Halton, with how big he is and how much power he has, he just put the bat on it and the ball just shot out of here. Both those pitches were up and both those pitches were home runs. I just can't make those mistakes."
Milwaukee added a run in the fifth when reliever Alberto Cabrera hit Norichika Aoki, Ramirez walked and Gomez followed with an RBI double. Davis walked to load the bases again for Halton, but he struck out to end the inning.
Ryan Sweeney was the only Cubs runner to reach second base with a double in the seventh off Nelson. Sweeney also had a single and Nate Schierholtz had the other two hits. The Cubs have lost seven of their last eight and are just 15-34 since July 29.
Halton's grand slam was his second key home run this week. His three-run walkoff homer on Sunday gave the Brewers a 6-5 victory over the Reds.
"It is just one of things," Halton said. " In a couple months I will sit back and look (and say) that was a crazy week. It is unbelievable to get an opportunity to be in those situations let alone come through in them."