The Giants battery of Matt Cain and Buster Posey made sure the Giants would stay in first place Tuesday night.
Cain and three relievers combined on a two-hitter, while Posey homered to lead San Francisco to a 1-0 win at Wrigley Field.
Cain and Cubs starter Carlos Zambrano each pitched six scoreless innings, but the Giants did just enough to prevail. Leading San Diego by a half-game and Colorado by 1 1/2 when the night began, San Francisco finally broke through with one out in the eighth when Posey drove a 3-1 pitch from Andrew Cashner to center field, pumping his fist as he rounded the bases.
He now has 15 homers, the most by a Giants rookie since Chris Brown hit 16 in 1985. His 62 RBIs are the most by a San Francisco rookie since Chili Davis' 76 in 1982. Throw in a .324 average, and it's easy to see Posey winning NL Rookie of the Year.
"Catching a shutout is probably as big a thrill as anything," said Posey, who also threw out Starlin Castro trying to steal second in the first inning.
"You can't get better than that," manager Bruce Bochy said.
In a game that was delayed for more than an hour by rain at the start, Cain and Zambrano sailed along before being lifted for pinch hitters.
The only hits the Cubs got were a single up the middle by Kosuke Fukudome in the second and a check-swing single to single to shortstop by Blake DeWitt in the sixth.
Ramon Ramirez (1-3) worked a perfect seventh for San Francisco, and Sergio Romo retired the side in the eighth. Brian Wilson pitched a perfect ninth for his major league-leading 44th save in 48 chances, leaving him four shy of Rod Beck's single-season club record.
"This is exciting," Cain said. "We're all having a lot of fun."