SUDBURY — After watching younger brother Mason come up huge with the bat all day on Monday against Chelmsford, Lincoln-Sudbury’s Cooper Tarantino one upped his sibling in a huge way.

After seeing Chelmsford turn a 2-0 deficit into a 3-2 lead in the top of the seventh, the Warriors pulled off a stunning rally in the home half, capped by Cooper Tarantino’s walk-off grand slam that gave L-S a 7-3 win and a date with No. 3 Xaverian on Wednesday in round two of the Division 1 tournament.

“I’m not really surprised to see Cooper do something like that,” Warriors coach Matt Wentworth said. “I think he’s the best hitter in our league and he hits the ball hard every single time he’s up there.”

The older Tarantino wouldn’t have had a chance to play hero if not for his younger brother’s perfectly timed long ball with one out in the seventh. The sophomore’s first career home run came off Chelmsford ace Matt Stuart, who had shut down the Warriors for most of the day.

“Funny thing is I woke up this morning feeling like I was going to get a couple of big hits,” Mason Tarantino said.

With the game now tied at three, Matthew Stagno singled and took second on a wild pitch with the top of the L-S (17-6) lineup due.

Jake Haarde was intentionally walked before Logan Umbach reached on an infield single to load the bases. All Cooper Tarantino had to do was get the ball out of the infield and he did more than that with his shot over the right-center fence.

“When you come up in that situation you know all you have to do is get the ball in play,” Cooper Tarantino said.

The frantic finish was in stark contrast to how most of the game went as Stuart and Warriors starter Matt Mahoney were taking turns putting zeroes on the board.

In fact, the teams combined for only six hits over the first six innings

The only blemish on either side came in the bottom of the third when Chelmsford (10-11) made a pair of errors that led to two runs for the Warriors. Ironically, it was a double from Mason Tarantino that got the inning started.

Mahoney breezed through the first six frames before the Lions offense came alive in the seventh.

Stuart led things off with a single. An infield hit by Grant Englehardt followed before a bunt moved the tying runs into scoring position. Mahoney then rallied from a 3-0 count to whiff Finn Ramseyer for the second out.

Keegan Briere’s infield hit to the hole at short brought pinch runner Jack Latham home to cut the lead to 2-1. No. 8 batter Matt Hall then gave Chelmsford its first lead when he doubled just inside the third base bag, scoring Briere and Englehardt to make it 3-2.

“It’s always like this in the first round of the tournament,” Wentworth said. “Everyone has that one guy who is lights out. We were fortunate that we had the last at bat and could rally.”

Lincoln-Sudbury’s Jackson Immonen is safe at third under the tag attempt by Chelmsford’s Keegan Briere. Lincoln-Sudbury rallied to post a thrilling 7-3 victory in state tournament action. (Chris Christo/Boston Herald)