SAN FRANCISCO — Conor Gillaspie never looked up to track the ball. The moment did not allow it. He was too consumed with a seemingly impossible task.

Not long after he helped the Giants topple the Cubs, 6-5, in 13 innings in Game 3 of the National League Division Series Monday night, all he recalled about his latest bit of October heroics is that he got his barrel on a 102-mph fastball thrown by the Cubs’ Aroldis Chapman.

He couldn’t recall the details — where the ball landed (the grass in right-center) or where it had crossed the plate (over the middle, just beneath the letters on his uniform).

“I knew I hit it good,” Gillaspie said, whose two-run triple in the eighth put the Giants in the lead for the first time. “I had no idea where it was. It sounds weird, but I literally had no idea where the ball was.”

Gillaspie, 29, added another chapter to his growing postseason legend. Last Wednesday, he sent the Mets’ season crashing down in the wild-card game, hammering a 96-mph fastball off Jeurys Familia for a three-run homer in the ninth. It was the only swing that mattered in the Giants’ 3-0 triumph at Citi Field.

On Monday, Gillaspie used that same, simple, lefthanded swing to overcome another elite closer in Chapman, though his odds in this confrontation were even more daunting.

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“Just fire your hands, you know?” Gillaspie said. “Like I said, sometimes it works out. Sometimes, it doesn’t. Obviously, it happened at a great time. We had to score.”

Gillaspie’s odyssey around the big leagues has been due partly to his struggles against lefthanders. Against them, he is hitting just .203 lifetime. Chapman has held lefties to a remarkable .122/.237/.155 slash line in his career, and Gillaspie was the first lefthanded batter ever to hit a triple off him. Until Monday, Chapman had permitted only nine extra-base hits to lefties.

When Joe Maddon summoned Chapman in the eighth — leaning on the closer to record a rare two-inning save — Giants manager Bruce Bochy had few choices. He burned righthanded-hitting Kelby Tomlinson earlier, and hitting for Gillaspie with anybody else would trigger a chain reaction of other difficult choices for a bench that was already short.

“But what Conor’s been doing for us, I didn’t think about hitting for him, to be honest,” Bochy said after Gillaspie came through.

Gillaspie’s done that plenty of late, especially down the stretch of the regular season, as the Giants attempted to steady themselves from a near-collapse that knocked them off their perch atop the NL West and nearly cost them a postseason spot.

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“There’s no surprise because he is so well prepared and he works as hard as he does,” said Joe Panik, who delivered the winning double in the 13th. “And he can hit a fastball, he’s a very good hitter. He’s proven that time and time again, and now he’s come up big for us all year.”

Gillaspie was a first-round pick of the Giants in 2008 and returned to his original organization only after signing a minor-league deal in the offseason. He was coming off a mostly unproductive stint last season with the Angels, who had acquired him from the White Sox.

In 101 games this season, Gillaspie hit .262 with six homers and 25 RBIs, for the Giants, world champions in 2010, 2012 and 2014, partly because they have won 10 straight elimination games.

“Everybody would agree that you love seeing any of your teammates have success,” Giants catcher Buster Posey said. “Especially a guy like Conor who’s had some ups and downs. He’s kept on pushing. He works really hard and he’s just an all-around nice person. So, it’s good to see him succeed.”