Morse displayed the Giants' revived penchant for slugging in their half of the third inning. Following the first of Hunter Pence's three hits, a leadoff double, Morse trimmed the deficit to one run by muscling Nathan Eovaldi's 99-mph fastball into the right-field arcade for his team-leading 10th homer.
Morse became the first Giant to amass 10 homers in the team's first 42 games of the season since Barry Bonds totaled 11 in the same stretch in 2007. Morse's homer happened to be the Giants' 50th of the season, a total they didn't reach last season until 75 games had elapsed on June 23.
The Giants forged ahead with a three-run fifth inning that was fueled by another opposite-field hit, Buster Posey's two-run double to right-center field. Morse, who matched Pence with three his, collected his third RBI by delivering Posey with a single to center.
Afterward, the joy in the clubhouse was reserved mostly for Cain, who recorded his first victory since last Aug. 17, at Miami, ending a winless streak of 11 starts.
"It had been a long time," Cain said.
His teammates registered no surprise over his ability to recover from his early struggles.
"The guy's awesome," Morse said. "To me, he's one of the best pitchers there is. He hasn't done anything I wouldn't expect him to do."
"I can tell you he warrants respect for a reason," said right-hander Sergio Romo, who pitched a perfect ninth for his 14th save in 15 chances. "For him to go out and find a groove -- [the Marlins] came out swinging, so you have to give them credit. But Matt definitely bore down when he needed to and almost got through eight [innings]."
Cain missed his target on the pitches that Derek Dietrich and Garrett Jones mashed for home runs in the first and second innings, respectively.
"If your timing's off, in general, you can miss by a lot," Cain said.
Morse didn't miss when he redirected Eovaldi's 1-1 heater.
"He's supplying all the power," Morse said. "I got a good pitch to hit and put a good swing on it. If it's going to go out, it's going to be that one."
The National League West-leading Giants, who have won eight of their last 10 home games, improved to 11-3 in series openers. Having won 16 of their last 21 games overall, they are building a collective confidence that's almost palpable. Rallying from Thursday's early deficit was the latest example of this.
"We just keep fighting, man," Morse said. "Our team's determined. We've got a good ballclub. We're not going to give in."
And if the Giants need a little luck occasionally, so be it. Good fortune was theirs in the fifth inning, when Pence topped a dribbler up the first-base line. The ball was ruled foul after Pence inadvertently kicked it in that direction, but replays showed that Pence was running in fair territory when he booted the ball, so he should have been called out. The play was not subject to video review, and with his at-bat prolonged, Pence singled, setting up Posey's go-ahead hit.
The Marlins tempered their complaints.
"An out is an out, and they are tough to come by in this league," manager Mike Redmond said. "It would have been a big out for us. The aftermath was three runs. Nobody saw it, and foul balls are not reviewable."
'That was frustrating," Eovaldi said, "but at the same time, I have to be able to move past that and make the next pitch."