Hunter Pence was looking to remain hopeful even as Giants manager Bruce Bochy was preparing to lose the outfielder to the disabled list.
Pence hobbled off the field with a right hamstring strain before the Giants blew a ninth-inning lead and lost to the Atlanta Braves 5-4 in 11 innings on Wednesday night.
Freddie Freeman led off the 11th with a homer to right-center off Derek Law (1-1).
Giants closer Santiago Casilla couldn't hold a 4-3 lead in the ninth.
Pence pulled up about halfway down the first base line while running out a slow grounder to third base in the fourth inning. Pence grabbed the back of his leg as he leaned over in pain before being helped off the field.
"He's probably heading to the DL," Bochy said.
Pence had a more positive outlook.
"I'm feeling optimistic," Pence said. "I'm able to walk on it pretty good right now. We don't know what it is just yet."
Pence said he will have a MRI on Thursday morning.
Pence returned to the lineup on Saturday after not starting in six straight games with a right hamstring strain. Bochy said this injury is in the same area.
San Francisco placed outfielder Angel Pagan on the 15-day disabled list on May 24 with a left hamstring injury.
Losing Pence for at least two weeks would be a blow for the NL West-leading Giants. Pence is tied for the team high with seven homers and leads with 36 RBIs. Jarrett Parker replaced Pence in right field.
"It's adding on to a tough loss to lose him," Bochy said. "And he's been feeling good."
Brandon Belt hit a two-run homer for San Francisco.
The Braves have won four of their last six in this 10-game homestand, including two of three against the Giants. Their 6-22 home record remains the worst in the majors.
"Obviously you don't want to get off to the start we did this season," Freeman said. "To finally kind of turn it around a little bit at home, where we were struggling the most, it's nice."
Freeman hit a hanging curveball from Law for the third game-ending homer of his career.
Chris Withrow (1-1) pitched a perfect 11th inning to pick up the win.
With one out in the ninth, Casilla hit Adonis Garcia with a pitch. Garcia moved to third when Nick Markakis singled to center field and scored the tying run on Casilla's wild pitch to Kelly Johnson.
Giants rookie Albert Suarez allowed three runs and three hits in five-plus innings in his first start in the majors. Suarez drove in a run with his first hit, an infield single in the sixth.