BOSTON — David Ortiz apparently isn’t too fond of surprises. But he got quite a big one Monday at Fenway Park.

Before Ortiz’s final home opener of his career, his 15-year-old daughter, Alex, completely surprised the Red Sox slugger by coming out to sing the National Anthem before Boston’s game against the Baltimore Orioles.

According to Alex, her father really didn’t have any idea what was coming, as she went out of her way to prepare while Ortiz wasn’t looking.

“Basically, what we tried to do was surprise him in the sense that, whenever I would warm up, he wouldn’t be there,” Alex explained to a small group of reporters after the great moment. “Also, we got out of school early, which was also an upside. We had to make sure that he knew that it was because we wanted to see the game.”

“I’ve never done anything in front of such a large group,” Alex said of singing the anthem. “I just kept thinking, ‘Focus on your breathing,’ because I really didn’t know what to do. … It honestly happened so fast. I only have hearsay to guide me in what actually happened after.”

What happened was that Alex’s powerful rendition of the “The Star-Spangled Banner” moved her All-Star dad to tears. The two shared a warm hug after her anthem, at which point Ortiz delivered an important message to his daughter.

“He told to me not to surprise him ever again,” Alex said. “I was like, ‘OK.’ ”

The special family moment was part of a memory-filled day for Ortiz. He threw out the ceremonial first pitch with a trio of Boston sports legends and, in his first at-bat — with his daughter watching on a press box television — ripped an RBI single off the Green Monster in left field.

“I think it’s going to click at, like, the 80th game of the season,” Alex said about the emotions of her father retiring after this season. “Otherwise, I don’t really know. We’ve been talking about it. He’s always like, ‘I’m done. I’m done.’ It’s probably happened for what, like five years now? But this year, it’s actually happening.

“I think we’ve been ready, slowly, but it’s going to be very difficult. We’re going to have to be around baseball. It’s his life.”

Thumbnail photo via David Ortiz hugs his daughter, Alex after she sings the National Anthem.