The Phillies remain 2½ games behind Atlanta in the East.
Rollins hit a three-run homer off of Cain in the fourth inning to give the Phillies a 5-1 lead. Rollins went 3-5 on the night, with 3 RBIs, just a double shy of hitting for the cycle.
"The last couple of days, I felt better," said Rollins, who missed 54 games this season with a calf injury. "My hitting area is starting to come to me. It doesn't feel like the plate is in the other batter's box."
"When there's a lot of focus on our team, Jimmy rises," manager Charlie Manuel said. "This is when Jimmy likes to play, or it seems that way."
The Giants have lost three straight to key opponents.
Cain gave up five runs -- two earned -- and seven hits in six innings. Giants starters are 0-9 with a 5.73 ERA in the last 14 games.
"We just have to think of the positives and get something going," Cain said.
"We're not scoring a lot of runs," Giants manager Bruce Bochy said. "We've got to get our offense going. There's a lot of baseball left. This team's been resilient. We'll bounce back."
On the bright side for the Giants, Andres Torres and Pat Burrell remain hot, each with home runs in the loss.
Torres crushed his 12th homer into the second deck in right field leading off the game and Burrell cut the lead to 5-2 in the sixth with his 12th homer in 58 games with the Giants. He has homered in his first two games back at Citizens Bank Park since helping the Phillies win the 2008 World Series.
Joe Blanton (5-6) pitched effectively, going 6 1-3 innings, and allowing two runs on eight hits with seven strikeouts.