SAN FRANCISCO -- During their pregame hitters meeting on Monday, the Giants played video of Mike Yastrzemski’s memorable walk-off grand slam off Josh Hader two years ago.

Yastrzemski looked like he might be in line for more heroics when he was due to lead off the bottom of the ninth against Hader with the game tied, but he ended up ceding the moment to Austin Slater, his platoon partner in right field. Slater typically pinch-hits for Yastrzemski against lefties, but he was still mildly surprised when bench coach Ryan Christenson told him to be ready to come off the bench in that spot.

“I was like, ‘Ryan, you sure about this? Yaz did just take him deep,’” Slater said. “And he was like, ‘Yeah, we’re sure about this.’ That was a nice supportive moment and something I appreciate.”

Slater wasn’t able to come through against Hader, popping up a down-the-middle sinker for the first out of the ninth, but he rewarded the Giants’ faith in extra innings, delivering an RBI single to cap a three-run rally and lift San Francisco to a 4-3 walk-off win over the Astros in Monday night’s series opener at Oracle Park.

The Astros scored two runs off rookie Erik Miller to go ahead, 3-1, in the top of the 10th, but the Giants rallied against right-hander Rafael Montero in the bottom half of the inning. Brett Wisely led off with a single that scored automatic runner Casey Schmitt from second base, and Heliot Ramos kept the line moving by reaching on an infield single.

That brought up Patrick Bailey, who lined a game-tying single to center field. The Giants then caught a break when Montero misplayed Michael Conforto’s tapper back to the mound, loading the bases with no outs. Two batters later, Slater ended the game by lining a misplaced sinker off the left-field wall for his second career walk-off hit and his first since April 8, 2022, against the Marlins.

Yastrzemski was one of the first players who emerged from the dugout to mob Slater, who had entered Monday batting only .156 with a .477 OPS over his first 27 games of the season.

“It felt great,” Slater said. “I was just trying to get the ball in the air, stay short and simple. Obviously, whenever you can come through big for your team in a spot when you’ve been struggling, it feels that much better. That definitely lifted a big weight off my shoulders, so that felt really nice.”

It’s been a challenging year for Slater, who underwent right elbow surgery over the offseason and felt like he may have “lost a little bit of the edge” after receiving limited at-bats during Spring Training. His season took another brutal turn when he missed 22 games with a concussion, though the injury gave him a chance to mentally reset and focus on regaining his timing at the plate during his rehab assignment with Triple-A Sacramento.

“I was able to go to Sacramento and kind of get my swing back and my timing and rhythm and came back with a lot of confidence,” said Slater, who is 3-for-8 in his first three games back from the injured list. “It’s a long season. When you look up at the board and your numbers aren’t great from the start, maybe it feels a little bit worse, but you go through stretches like that all the time in the season. I’m just trying to keep that mindset that it’s a long season and take it one at-bat at a time. That definitely helps.”

Slater’s game-winning hit ensured that the Giants didn’t waste a gem from left-hander Kyle Harrison, who held the Astros to one run on four hits over 6 1/3 innings. Harrison struck out three and walked none in the efficient 77-pitch outing, lowering his ERA to 3.96 over 14 starts this year.

“This was just his best stuff we’ve seen in a while today,” manager Bob Melvin said.

Yastrzemski briefly gave Harrison a 1-0 lead by driving in Jorge Soler from first base with a 406-foot triple off Spencer Arrighetti in the bottom of the sixth, but the Astros tied the game after Yordan Alvarez tripled and scored on Jeremy Peña’s sacrifice fly in the seventh.

Despite Yastrzemski’s big hit -- and his impressive full-extension catch to rob Jon Singleton in the eighth -- Melvin didn’t hesitate to lift him for Slater in the ninth, which ended up working out well with the Giants in the long run.

“That was huge,” Melvin said. “It’s been a little uneven from the very beginning with the injuries and a little bit of a struggle to start and not getting a ton of playing time. … He put a pretty good charge into that one, so it’s pretty rewarding for him after all the hard work.”