Right-hander Bud Norris can finish off a three-game series game set at Minute Maid Park.
On the short end of a 4-3 decision in game one on Monday, the Astros leveled the series in stirring fashion Tuesday when Brian Bogusevic hit a game-winning pinch-hit grand slam in the bottom of the ninth inning for a 6-5 win.
Bogusevic ended the game by driving a pitch over the wall in right-center to cap the wild comeback.
Aneury Rodriguez (1-4) recorded the final two outs in the top of the ninth.
Jimmy Paredes finished with two doubles and drove in a run for the Astros, who won for only the second time in 12 contests.
Starter Brett Myers allowed six hits and four runs over seven full frames, but was spared his seventh straight negative decision thanks to the unlikely finish.
Tyler Colvin hit a two-run homer and Aramis Ramirez added two hits -- including a solo shot -- and two RBI for the Cubs, who lost for just the fourth time in their last 16 games despite being well out of playoff contention and mired in fifth place in the NL Central.
Chicago hurler Ryan Dempster was on pace to win his fourth consecutive start, going seven innings and yielding just four hits and two runs with three walks while fanning seven.
Norris, a 26-year-old Texan, has been among the few bright lights in a difficult season for the Astros, buried deep in last place in the six-team NL Central.
The nine-game winner from 2010 has lowered his earned run average by nearly 1 1/2 runs thus far in 2011, allowing 58 earned runs in 147 innings after giving up 84 in 153 2/3 innings last season.
It's not translated into victories, however, as Norris is 0-2 with four no- decisions since his last triumph, an 8-2 win at Pittsburgh on July 6.
He pitched seven scoreless innings for a tough-luck no-decision in his last start, allowing two hits and striking out eight in Houston's 1-0 loss to the Los Angeles Dodgers.
Norris has given up three runs or less in six of his last 10 starts, but has won just once.
He dropped a 4-2 verdict to the Cubs in Chicago on July 22 and is 1-2 against them in four career appearances.
He's opposed by 24-year-old righty Casey Coleman, who pitches for the first time since a 5 1/3-inning appearance against Washington on July 4.
The Florida Gulf Coast University product is getting another chance in the rotation thanks to the antics of Carlos Zambrano, who was placed on the 30-day disqualified list after being tossed from a game last week and clearing out his locker.
A four-game winner in his rookie audition last season, Coleman won his initial decision of 2011 against Colorado on April 16, but was subsequently mediocre in three straight losses through mid-May.
He beat Florida, 5-1, with 5 2/3 innings of scoreless ball on May 19, then retired just four batters against the New York Mets and made two bullpen outings before the aforementioned holiday start against the Nationals and a subsequent trip to the minors.
Coleman is 1-0 in two starts against Houston with a 2.77 ERA in 13 innings.
The Cubs have won six of 11 over the Astros this year.