After Sosa had already hit his major league-leading 53rd home run in the fifth inning, Baker chose to face Grace.
"He made the wrong (decision)," Grace said.
The Cubs trailed 7-2 after three innings before rallying behind Sosa and Grace, who hit consecutive solo homers in the fifth. Sosa has 10 homers in the last 12 games for the Cubs, who are just 5-20 in August.
"We felt like we won the World Series, just winning that game," Grace said.
Sosa, who has nearly one-third of the Cubs 160 homers, connected on a 1-1 pitch from Livan Hernandez in the fifth. Sosa is nine games ahead of last year's pace, when he hit No. 53 in the Cubs' 135th game and ended with 66.
"It's not for me to think about the race," said Sosa, whose 2 1-2 year old son Sammy Jr. hugged him in the clubhouse after the game. "All I can do is go out and be patient."
Sosa is getting close to being the first player with two 60-homer seasons.
"It's easy this year after last year and everything I've been through," Sosa said of all the attention. "This is a piece of cake."
The Cubs won the game in the ninth. Jeff Reed hit a leadoff double off Robb Nen (3-7) and two outs later Morandini doubled to left-center to give the Cubs their first win in eight games this season against the Giants.
"Obviously, we've been struggling and we need wins and to be down so many runs and come back and win that game, it was a lot of fun today," Morandini said.
Terry Adams (5-3) pitched one scoreless inning.
Grace added a pair of two-run singles to match his career RBI high of five. His second two-run single tied the game in the eighth inning against Nen.
"I blame myself," Nen said. "My job is to save games. When you've been as inconsistent as I have, you can't do it."
The Giants intentionally walked Sosa with two out and two runners on in the eighth to load the bases. Grace, who was 1-for-13 against Nen entering the game, followed with a single to right.
"They're not going to let me beat them in that type of situation," Sosa said.
"When the other team shows you up like that, you've got to take it personally," Grace said. "It's my job to protect Sammy."
Jeff Kent hit a two-run single in a six-run third and a two-run double in the sixth, and has hit safely in all five games played since being activated from the disabled list Saturday.
Rookie Micah Bowie gave up seven runs six earned and seven hits in 2 1-3 innings for Chicago. It was his shortest outing in five starts.
Trailing 2-1, San Francisco scored six runs in the third inning. Kent hit a two-run single, J.T. Snow hit a two-run double, Scott Servais added an RBI single and another run scored on Livan Hernandez's sacrifice.
Kent hit a two-run double in the sixth and scored on Snow's single to open a 10-6 lead.
"I'd be happy as heck about 6-1 (road trip), but I'm not because we had a lead today," Baker said. "When you're one out away from winning, that's what hurts."
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