SAN FRANCISCO -- Tyler Walker owns solid credentials as a reliever. He has set up 16 of Brian Wilson's National League-high 23 saves, yielding just one run in those appearances. He hadn't allowed a run in eight consecutive outings entering Wednesday. Opposing right-handers are batting .174 off him.

But Walker's performance against left-handed batters is threatening to undo his otherwise capable body of work.

Left-handers are batting .364 (16-for-44) this year against Walker. The right-hander's tormentors include Mike Fontenot, whose tiebreaking home run in Wednesday night's eighth inning lifted the Chicago Cubs to a 6-5 victory over the Giants.

This game could have gone a long way toward improving the Giants' fortunes at home, where they continue to grope for consistency. So the Giants can't generate enough power to overcome AT&T Park's dimensions? Rubbish. John Bowker's two-run, sixth-inning homer wiped out most of a 3-0 deficit before Ray Durham launched a three-run homer to forge a 5-5 tie in the seventh inning.

That's when the dramatics disappeared for the Giants, although the long ball remained a key element in the outcome. Fontenot ensured that by driving a 3-1 fastball onto the right-field arcade with one out in the eighth for his fifth homer of the year.

As a result, the Giants haven't won two home games in a row in the same series since May 10-11 against Philadelphia. They captured back-to-back games on June 1-2 against San Diego and New York, respectively. Since defeating the Mets, the Giants have dropped nine of 11 games at AT&T Park, contributing to their Major League-worst 15-26 mark at home.

The Giants could entertain thoughts of a better fate, particularly after Durham delivered his drive off Carlos Marmol (2-3). Pinch-hitter Travis Denker's two-out walk prolonged the seventh for Fred Lewis, who singled in front of Durham's third homer of the season.

Then came Fontenot's homer, prompting plenty of self-examination from Walker. Entering this year, left-handers had hit .282 off him -- nothing phenomenal, but better than he has fared this season.

"In the past I've been able to get lefties out pretty well. I get the ball away from them, get it off the barrel of the bat and take some of the sting out of it," Walker said. "It seems like this year I haven't been able to locate that ball away to lefties. The two-seamer's not really moving; it's staying right down the middle. It's not a good situation, especially when you're dealing with Major League hitters who are going to make you pay for it."

Of the five homers Walker has surrendered this year, four were struck by left-handed batters. St. Louis' Rick Ankiel and Chris Duncan and Houston's Darin Erstad preceded Fontenot.

"Lefties are seeing the ball well off me, executing their swings and hitting the ball," said Walker (3-4). "It's an adjustment we're going to have to make and we're going to have to make it quickly."

Giants manager Bruce Bochy defended Walker. "I think Tyler has done a great job lately," Bochy said, pointing out that Walker retired Eric Patterson and Kosuke Fukudome, both left-handed hitters, after Fontenot went deep. "He just made a mistake there."

Walker wasn't the only Giant who left AT&T Park feeling haunted. Center fielder Aaron Rowand went 0-for-4 as his average dropped to .290, the lowest it has been since April 18 (.275). Rowand struck out three times, including a punchout that ended the third inning with the bases loaded, and is hitting .190 (19-for-100) in his last 26 games.

"Everybody has their ups and downs, but that's the way this game goes," Bochy said. "Aaron Rowand will be fine."

It was an encouraging evening for Kevin Correia, who was charged with Chicago's first four runs but worked 6 1/3 innings, his longest outing in four starts since returning from the disabled list. Correia's primary nemesis was Jim Edmonds, who drew a leadoff walk and scored in the second inning before hitting a two-run, opposite-field homer to left in the sixth.

"I think my changeup has kind of been letting me down," Correia said. "It's one of my better pitches and, with Edmonds, I got ahead of him and went to the changeup and ended up walking him because I couldn't throw a good one. [In the sixth inning] I threw him a 2-2 changeup to go to 3-2, and a 3-2 changeup that he hit out."

Bowker's homer, a "splash hit" into McCovey Cove, was the second hit into the bay this season by a Giant. Lewis preceded Bowker on April 26 against Cincinnati. They are among nine Giants not named Barry Bonds to reach the water on the fly since the park opened in 2000.