Matt Garza was sailing along when third baseman Joe Mather booted a routine grounder in the sixth inning.
Then it all fell apart in a hurry for the Cubs.
Mather's error sparked a four-run inning for the Detroit Tigers, who rallied for an 8-4 victory on Wednesday night.
"I just missed it," Mather said. "It was the turning point in the game. Garza got out of trouble right there. He was in a little bit of trouble and he got out of it. I feel bad that that's what kind of started it all."
Brennan Boesch had four hits, including a solo homer, and made a diving catch in right field as the Tigers improved to 8-1 in their last nine games against the Cubs. The one loss during that stretch occurred in the series opener Tuesday night, when shortstop Jhonny Peralta made two errant throws in the eighth inning that led to the decisive run for Chicago in a 4-3 victory.
But Peralta responded with three hits and two RBIs in the middle game of the series, and the Tigers will send Justin Verlander to the mound Thursday afternoon with a chance at a heartening 4-2 interleague road trip for a club that has underachieved so far this year.
"Tomorrow's a big game for us," manager Jim Leyland said, "because you go to Cincinnati and Chicago and you've got a chance to win both series. That's pretty good. You've got to try to win each series. If you can do that, then you put yourself back in decent shape."
With one out and runners on the corners in the sixth, Peralta hit a drive to center for a two-run double that tied it at 4. He advanced on Gerald Laird's bunt single against Garza and scored on Ramon Santiago's comebacker to put Detroit in front.
Boesch added his seventh homer in the seventh and an RBI single in the eighth, delighting the sizeable portion of Tigers fans among the season-high crowd of 41,326 at Wrigley Field.
"I think that's a tribute to Detroit fans," Boesch said. "The road fans are my favorite fans and most of the players' favorite fans because they come and they bring the noise, bring the passion. They're die-hard Tigers fans."
Alfonso Soriano, Darwin Barney and Steve Clevenger had two hits apiece for the Cubs, who had won four straight home games. Mather, inserted into the lineup when third baseman Ian Stewart was scratched with a sore left wrist, contributed an RBI single but his error put two runners on with no outs and Garza wasn't able to recover.
"It was too bad, because obviously he had good stuff and was cruising through that game," Cubs manager Dale Sveum said. "Got that double-play ball, obviously changed the game around."
Garza (2-5) allowed five runs, three earned, and seven hits in six innings. The right-hander is 0-4 with a 5.26 ERA in his last seven starts.
"It is what it is. Same old stuff," he said. "Just going to keep getting ready for the next one. Felt great tonight, felt I could have went seven, eight, and further. I had good stuff. Just stay around that and keep going. Control what I can control."
Soriano and Bryan LaHair hit consecutive run-scoring doubles to make it 4-1 in the fifth, but the Tigers eventually solved Garza on the way to their third victory in the last four games.
Rick Porcello (4-4) gave up nine hits in five innings, but managed to earn his first win since May 6 against the White Sox. The right-hander was 0-2 with a 5.17 ERA in his previous six starts.
The Cubs and Tigers exchanged two fielding gems in the seventh. Shortstop Starlin Castro made a diving stop on Miguel Cabrera's smash in the top half of the inning, but Boesch robbed David DeJesus of a hit in the bottom half.
Detroit center fielder Austin Jackson added a great catch against the wall in center to end the game.
NOTES: Stewart is going to visit another doctor on Thursday, and Sveum said he was headed for the disabled list. Sveum said he wasn't sure yet which player was coming up from the minors. ...1B Anthony Rizzo began the day with a .367 batting average, 22 homers and 57 RBIs for Triple-A Iowa, and Sveum acknowledged he could be promoted soon. "The name is there and it's obviously getting built up to getting to that point where it's getting near now," Sveum said. Rizzo left Tuesday's game with a bruised right knee, but Sveum said he's fine. ... Verlander (5-4, 2.69 ERA) will face LHP Travis Wood (0-2, 4.71 ERA) in the series finale. Verlander pitched 5 2-3 innings of one-run ball in his only start at Wrigley Field, a 9-3 victory on June 17, 2006. He is hitless in 21 career at-bats with 13 strikeouts. Wood is searching for his first win since he recorded two outs for Cincinnati at Pittsburgh on Aug. 19.