Lance Berkman's go-ahead, two-run homer in the eighth inning wasn't enough. Lidge blew his first save in more than three months, allowing two runs and four hits in a 4-3 loss to the Chicago Cubs on Friday night.
"If we had won 10 in a row to get a one-game lead right now, I'd be happy as all get out," Astros manager Phil Garner said. "No, I don't like the way that game turned out, but we still control our destiny."
The Cubs, who won the series opener Thursday night, started the ninth with three consecutive hits off Lidge (4-4), who had converted 24 consecutive save chances since June 11. After Matt Murton and Jeromy Burnitz hit consecutive singles, Michael Barrett doubled off the top of the left-field wall to tie the game at 3-all.
"They did a good job hitting mistakes. Obviously, it's a big disappointment," said Lidge, who has 40 saves. "I felt good, but I couldn't get on top of my slider. I don't know if it was my mechanics or not. There's no excuse. I just made bad pitches."
Corey Patterson hit a hard chopper to first, and Berkman gathered the ball and threw home for the first out. But pinch-hitter Todd Walker, playing for the first time since hurting his knee Sept. 23, hit a grounder by Berkman for a single that sent home Barrett with the go-ahead run.
"It's surprising, but he only gave up one decent hit," Garner said. "The others just rolled through the infield."
Philadelphia (86-74) won 4-3 at Washington and is just behind Houston (87-73) in the wild-card race.
Roger Clemens (12-8), who skipped his previous turn because of a sore left hamstring, is the scheduled starter for the Astros on Saturday, and Roy Oswalt (19-12) is set to go Sunday. If the Astros and Phillies finish Sunday tied, they would meet in a one-game playoff Monday at Philadelphia.
Lidge was in trouble at about the same time his predecessor, Billy Wagner, was closing Philadelphia's victory.
"It was shocking, but again, he's human," said Astros starter Andy Pettitte, who allowed one run in seven innings.
Dempster retired pinch-hitter Orlando Palmeiro on a flyout to right for his 33rd save in 35 chances, his 19th in a row. The Cubs would finish at 81-81 if they win the final two games.
"It doesn't matter if we're in the playoffs or not, we're trying to get to .500," Cubs manager Dusty Baker said. "We've got something to play for. Guys are still trying to get hits and get wins."
Berkman's 24th homer -- and 11th in September -- came with one out but wasn't enough to keep the Astros from wasting another impressive start by Pettitte.
After Pettitte was lifted for a pinch hitter in the seventh with the game tied, the Cubs took a 2-1 lead in the eighth off Chad Qualls. Neifi Perez had a two-out infield single, and shortstop Adam Everett bobbled Derrek Lee's grounder for an error.
Nomar Garciaparra doubled to put the Cubs ahead. Lee was thrown out trying to score after center fielder Willy Taveras, who had bobbled the ball, threw to Everett for the relay to Ausmus, who tagged out the sliding Lee.
Taveras led off the Astros eighth with a single off Roberto Novoa (4-5) and, after Morgan Ensberg fouled out, Berkman hit an opposite-field shot into the left-field stands. His 11 September homers tied the team record set by Richard Hidalgo in 2000.
Pettitte, who missed the Astros postseason run last season because of a torn tendon in left elbow, made his second start in a row against the Cubs in which he allowed just one run without a decision. Both were one-run losses. The left-hander had won seven consecutive starts before then.
Carlos Zambrano allowed a run and four hits over seven innings, but missed out on his 15th victory.
The Astros failed to score after Zambrano loaded the bases with consecutive walks to Ensberg, Berkman and Mike Lamb in the sixth. Jason Lane swung at the first two pitches, then grounded to Perez at shortstop.
Lamb, in the starting lineup against the left-hander, put the Astros ahead 1-0 in the second with his 12th homer.
Chicago tied the game in the sixth when rookie Ryan Theriot, making his second major-league start, doubled and scored on a sacrifice fly by Lee, who got his 107th RBI.
- Biggio hit his 603rd career double, 11th on the career list with Cal Ripken Jr.
- The Astros had a crowd of 41,304 for their 14th sellout this season, 10 fewer than last season.
- Berkman has 25 walks in September, his high for a month in his career.