Chicago, IL (Sports Network) - Buster Posey's solo homer was the lone run of the contest, as San Francisco edged Chicago, 1-0, in the opener of a three- game set from Wrigley Field.
Matt Cain got the start and limited the hosts to a pair of hits over six innings for the Giants, who have won four of six.
San Francisco stayed a half-game ahead of San Diego in the NL West, as the Padres downed the Dodgerslater Tuesday.
Ramon Ramirez (1-0) turned in a scoreless seventh frame for the win, while Brian Wilson picked up his 44th save with a perfect ninth.
Carlos Zambrano worked six innings, fanning eight with five walks and giving up only three hits for the Cubs, whose six-game win streak was halted.
Andrew Cashner (2-6) allowed Posey's homer over two relief frames for the loss.
Cashner retired Aubrey Huff to start the eighth, but Posey greeted the reliever with a long homer to center for a 1-0 Giants edge.
Sergio Romo followed by retiring the Cubs in order in the home half, then Scott Maine added a scoreless top of the ninth for Chicago.
Wilson finished the contest with a 1-2-3 home ninth.
Zambrano left the bases loaded in the second. Though Pat Burrell walked and Pablo Sandoval and Juan Uribesingled, Cain struck out to quell the rally.
The visitors threatened again in the fifth, but Posey flied out to right with the bases loaded.
Chicago left one on in the sixth after Marlon Byrd grounded into an inning- ending double play.
San Francisco has won four of five in the season series...Cain also held the Dodgers scoreless on three hits over seven innings in his last start on September 15...Zambrano had turned in 8 2/3 scoreless frames at Milwaukee during a September 10 victory...The start of the game was delayed one hour, 11 minutes due to rain.