BOSTON — David Ortiz has kept his cool during some of the biggest Red Sox moments ever. But even the greatest clutch hitter in franchise history couldn’t keep it together Monday.

Ortiz got quite the surprise at Fenway Park ahead of the final home opener of his career, as his 15-year-old daughter, Alex, came onto the field to sing the national anthem before Boston’s game against the Baltimore Orioles.

Alex said she didn’t tell her father of her plans, and Big Papi admitted he had no idea what was coming.

“It was a little bit of a surprise,” Ortiz said after Boston’s 9-7 loss. “Caught me off guard. My daughter, she always takes music lessons; she always sings and does that kind of stuff at home, and that’s cool. But I was pretty surprised. I was nervous. I was nervous. I was dying.”

Video cameras captured Ortiz tearing up as his daughter performed a pretty impressive rendition of “The Star-Spangled Banner.” So, just how important was Monday’s moment for a man who’s recorded too many walk-off hits to count?

“I’m not gonna lie. I was more nervous during that time than during any at-bat that I ever had in my career,” Ortiz said. “It wasn’t even about me, you know what I’m saying? It was about her. Whoever has kids, you know how that goes, when you are watching your kids performing or anything. And that was, like, my first big moment, watching one of my (children) just doing something pretty big.

“Now, I understand my dad and my family, my mom, when she used to watch me, I know they always used to be very nervous and stuff. And now I get it. But it was unbelievable, and hopefully it stays in everybody’s memory. It was a wonderful moment.”

Ortiz’s day at the plate produced mixed emotions. He smacked an RBI single off the Green Monster in the first inning and doubled in the seventh inning, but grounded into a double play in the ninth inning while representing the winning run at the plate.

But Monday went beyond baseball for the 40-year-old father of two, who also threw out the ceremonial first pitch alongside Boston sports icons Bill Russell, Bobby Orr and Ty Law. Watching his daughter sing by far was the highlight of his day, though.

“I got caught (up) into the emotions,” Ortiz said. “I was really nervous. It was a surprise, a beautiful surprise. At some point you start thinking about your kids, you know? You raise your kids and everything that you have been through, and I got caught into those memories, and it was beautiful.”

And perhaps best of all, the Red Sox slugger admitted his daughter’s performance in her first time in front of a large crowd was, for lack of a better word, clutch.

“She came through,” Ortiz said. “She did come through.”

Thumbnail photo via David Butler II/USA TODAY Sports Images

Thumbnail photo via Boston Red Sox designated hitter David Ortiz (34) takes the field before the Red Sox home opener against the Baltimore Orioles at Fenway Park.