The Pirates also fell three games behind the Cardinals for the second National League Wild Card spot. They've also been passed in the standings by the Milwaukee Brewers.
The loss -- given to right-hander Jared Hughes -- was the latest in a rough September for the Pirates' bullpen. Bucs relievers have a 5.81 ERA in 14 games this month, during which the Pirates are 3-11. Manager Clint Hurdle said he doesn't think his relieving corps is fatigued, but wouldn't disagree with those who believe it could be.
"With the numbers and the way we're playing out there, you might think that's it," Hurdle said. "But the appearances and the pitches aren't anywhere near normal redline numbers that you have."
There were plenty of cautionary numbers put up Sunday, most of them coming from Cubs first baseman Anthony Rizzo.
Rizzo hit two home runs, the second of which was his first career grand slam in the sixth inning that put the Cubs ahead, 10-9. His previous homer, a two-run shot off Locke, came in the four-run fifth that brought the Cubs within one.
"We were in the driver's seat that game," Locke said. "To give the other team like that the opportunity to put runs up on the board and eventually keep touching home plate, eventually that's going to come back to hurt you in a game. And today it did."
Rick van den Hurk began the decisive sixth inning by giving up consecutive singles and walking the base loaded before getting pulled for Hughes. Hughes allowed an RBI single to second baseman Darwin Barney before Rizzo blasted the righty's 2-1 sinker into the right-center field seats.
Van den Hurk -- making his second appearance of the year -- got the call from the 'pen instead of veteran right-hander Jeff Karstens, who hasn't pitched since leaving a Aug. 31 with right hip flexor discomfort. Hurdle said he thought about bringing in Karstens or right-hander Chris Leroux, but noted Karstens' early struggles this season. Karstens has allowed 15 earned runs in the first inning this year, a 9.42 ERA in that frame.
"I wanted to bring in sinkerball guys from that point on to try to hold us," Hurdle said. "The guy I felt that could give us some length and I chose van den Hurk. Didn't work."
Pirates left fielder Starling Marte led off the seventh with a triple, but the Pirates couldn't take advantage until a two-out walk by Garrett Jones. Cubs reliever Jaye Chapman faked a third-to-first pickoff attempt when Jones broke for second during the next at-bat and Marte took off for home, but Chapman held onto the ball as Marte was tagged out in a rundown.
Hurdle said he thought Marte might have been able to score had he been more patient, but the Cubs got out of the inning before adding three runs in the eighth.
"I looked at second base, and a thought ran through my head, 'Hey, that guy's going to score' and I turned around and sure enough, Marte had taken off,'" Chapman said. "I was like, 'Just make the throw and let's get out of here.'"
The bullpen blowup negated a big day for third baseman Pedro Alvarez and a Pirates lineup that displayed impressive speed. Alvarez homered twice and the Bucs stole a season-high four bases, two each by Andrew McCutchen and Marte. Alvarez's multi-homer game was his fourth of the season, while the two bags swiped by McCutchen and Marte equaled and set season highs, respectively.
The wraparound series at Wrigley Field figured to wake up Alvarez, who came to Chicago on an 0-for-14 skid. The third baseman has thrived in day games this season -- hitting .301 with a Major League-high 18 daytime home runs -- and continued that production this weekend.
Alvarez snapped his hitless streak Friday, had another knock Saturday and hit both his homers Sunday on Waveland Avenue.
Unfortunately for he and the Pirates, both went for naught.
"When you score nine runs you believe you did better than the result we got," Hurdle said. "At the end of the day, we've got to find a way to show up tomorrow, split the series and get back to the house and get ready for the Brewers."