It was Jason Diaz’s game from the moment he stepped on the mound at Hofstra’s University Field on Monday.

The Kellenberg sophomore showed poise in the biggest game of his first year on varsity, striking out 10 batters in a four-hitter as the top-seeded Firebirds defeated No. 3 St. John the Baptist, 1-0, in the CHSAA championship game.

The Firebirds (16-4), who won their second consecutive championship, did not allow a run in three playoff games.

Diaz, who walked two batters and threw 95 pitches, said he had goose bumps before the first pitch, but he shook off early nerves to execute his two-seamer, changeup and slider.

“I gave it all in those seven innings,” he said.

The game’s lone run came in the bottom of the first inning. Chris Cappas slapped an 0-and-2 pitch to leftfield for a single that drove in Andrew Russell, who had doubled to right-center to start the inning.

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Cappas was 2-for-2 with a walk and was named the CHSAA player of the year and pitcher of the year.

“We didn’t expect that at all,” Cappas said of the first-inning run being the difference. “We had a lot of chances, we just didn’t score. Their pitchers did a great job. So did their defense.”

St. John the Baptist starter Nicholas Jack allowed four hits and struck out three in 3 2⁄3 innings. He left with runners on first and second and two outs in the bottom of the fourth, and Bryan Krolikowski struck out the first batter he faced to end the threat. Jack had worked out of bases-loaded jams in the first and third for SJB (13-10).

Diaz lined into a double play with the bases full and one out in the first when Cougars shortstop Matthew Archer snagged a liner and fired to second. Diaz also popped to second with the bases loaded and two outs in the third.

“Even though we didn’t score the rest of the game, I knew my defense was going to back me up,” Diaz said. “So I had a lot of confidence.”

The defense did its part behind Diaz in the fifth. Leo Clarke led off and drove the ball to the warning track in left-centerfield. Leftfielder Mike Repoli, who was not in the batting lineup but played leftfield because of his defensive talents, dived and made a spectacular catch in front of the wall, robbing Clarke of a sure extra-base hit.

“The ball was carrying all day, so I figured I wasn’t going to get to it,” Repoli said. “But it was hanging up there, so I kind of just laid out for it and tried to make a good play on it.”

The Cougars didn’t go quietly. Singles by Ryan Watson and Clarke put runners on first and second with two outs in the seventh. But Diaz got a check-swing strikeout on a slider in the dirt, and catcher Jonah Katz threw to first for the final out.

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Katz and his teammates then celebrated with Diaz to the right of the mound — a mound Diaz dominated from the first pitch.