The St. Louis Cardinals wasted a golden opportunity to tighten the NL wild- card race after a ninth-inning collapse by their bullpen.
While the Chicago Cubs have little left to play for, they'd love to end their archrivals' playoff aspirations.
The Cardinals open a key three-game series with the Cubs on Friday night at Busch Stadium.
"We're trying to play our way into the playoffs and this is when you push," manager Tony La Russa said.
St. Louis, which committed two errors in the ninth, lost for the third time in 16 games. The Cardinals (86-70) trail wild card-leading Atlanta by two games with six left.
"Don't make a mistake and say we're heartbroken," La Russa pointed out. "Our heart's beating. We won the series, get ready for (Friday)."
St. Louis has won eight of 12 against Chicago this year and the bullpen has been especially strong in six home meetings, posting a 1.56 ERA while allowing nine hits, walking one and striking out 15 in 17 1-3 innings.
Chris Carpenter (10-9, 3.66 ERA) will try to give the bullpen a break as he looks to end a long winless stretch at home against the Cubs.
The right-hander is 3-3 with a 2.25 ERA in 10 career starts versus Chicago in St. Louis, but he's 0-2 with a 3.07 ERA in his last four such outings dating to Sept. 20, 2009.
Carpenter pitched well enough to beat the Cubs at Busch Stadium on June 5, allowing two runs over nine innings before St. Louis won 3-2 in 10. He was reached for four runs and a career-worst 13 hits - all singles - in seven innings of a 6-4 victory at Wrigley Field on May 10.
The 2005 NL Cy Young Award winner hasn't allowed a run in each of his last two wins. He tossed a four-hitter to defeat Milwaukee 2-0 on Sept. 7 and scattered eight hits over eight innings Sunday in a 5-0 win over Philadelphia.
Slumping Ryan Dempster (10-13, 4.63) will take the ball for the Cubs. The right-hander is 0-5 in his last seven starts.
Dempster has lost each of his last two starts, falling 3-2 to major league-worst Houston on Sunday. He was charged with three runs and seven hits in seven innings.
Dempster is 1-1 with a 7.36 ERA in two starts at Busch Stadium this year.
Pujols reached base safely for the 38th straight game Thursday, leaving him one shy of matching Tampa Bay's Johnny Damon for the longest run in the majors this year. Pujols, the NL home run leader, needs three more to reach 40 for the seventh time in 11 seasons.
Chicago (69-87) won for the fourth time in six games after Wednesday's 7-1 rout of the NL Central-leading Brewers.
Starlin Castro went 2 for 3, leaving him one hit shy of becoming the first Cub with 200 in a season since Juan Pierre in 2006. At just 21, he would also be the youngest of 16 players in franchise history to reach the milestone.
"We're all anxious to see it happen," teammate Carlos Pena told the Cubs' official website. "It's a very impressive feat to accomplish."
The second-year shortstop is 5 for 27 (.185) in six games in St. Louis this season but 7 for 18 (.389) when facing Carpenter.