ST. LOUIS — The Stanley Cup finals started with St. Louis Blues forward Zach Sanford watching the games rather than playing in them. It started with the Massachusetts native’s friends texting him that, as much as they were rooting for him, they might be rooting for their hometown Boston Bruins more.

It started with his mother sitting in the stands reminding herself to cheer when St. Louis did something good, rather than when Boston did, and with Sanford thinking of his father, as he often does, and what he would say about all of this.

It could end with the forward playing a pivotal role in St. Louis beating the team he grew up rooting for. His promotion into the Blues’ top-six forward corps has correlated with back-to-back wins that have St. Louis on the cusp of its first Stanley Cup.

Thursday’s Game 5 was his first NHL game at Boston’s TD Garden, and less than a minute into the second period, Sanford retrieved a puck behind the Boston net, drawing both Bruins defensemen with him. He backhanded it through his legs to center Ryan O’Reilly in front for the game’s first goal and Sanford’s third assist in as many games.

“It’s been a hell of a year, that’s for sure,” Sanford said with a chuckle.

His season has been about as dramatic as the Blues’ journey to this point — the league’s last-place team in early January is now one win away from the championship. His father, Mike, died suddenly of a heart attack during training camp, when Sanford was fighting to make the NHL roster. Then in December, Sanford got into a fight with teammate Robert Bortuzzo during practice. Although the team downplayed the incident, Sanford was sent to the Blues’ American Hockey League affiliate within the week, still not quite a full-time NHL forward.

After Sanford was a healthy scratch for most of the Blues’ postseason run, an injury to Robert Thomas and Oskar Sundqvist’s one-game suspension cleared a path into the lineup. His chemistry with O’Reilly and David Perron on the second line — the trio have four goals in the past two games — has helped tilt the series in the Blues’ favor.

“Since he’s been in, he’s made an impact — not only on the score sheet with making big plays and getting points but just overall wearing teams down,” O’Reilly said Thursday night. “He’s being physical at the right time or making plays and having that puck possession. He’s been a huge piece in us finding a way to create against this team.”

This time a year ago, Sanford watched from afar as the Washington Capitals, the team that drafted him, celebrated their first Stanley Cup, and he acknowledged that “maybe a little” part of him had wondered what might have been were it not for his February 2017 trade to St. Louis. In a midseason deal for defenseman Kevin Shattenkirk, Sanford, who had played in 26 games for the Capitals as a rookie, was shipped out as the centerpiece of the return for St. Louis.

But Sanford dislocated his left shoulder at training camp before the next season, and just as he had recovered, a bruised lung sidelined him again, further derailing a year in which he didn’t get any NHL playing time.

The 24-year-old played in 60 games this season, posting eight goals and 12 assists, but consistency still eluded him. This three-game stretch has been a window into what the Capitals had hoped Sanford would be when they used a second-round draft pick on him in 2013: He’s using his 6-foot-4 frame to be a physical force while still possessing the offensive and skating skills to contribute, as forward Tom Wilson has with Washington.

“There’s still a lot of stuff I need to work on, obviously, but a lot of guys want to tell themselves they’re a top-six guy,” Sanford said. “Those are the guys who get all the points and play the power play and this and that. But I don’t really try to think about that too much. I’ve just been trying to work on the things I need to get better at, and everyone says to stay good at the things that you’re good at. I think recently I’ve been doing a pretty good job at the things I’m usually pretty good at.”

Said Blues captain Alex Pietrangelo: “Even if he didn’t get a point, if you just watch what he did on the forecheck — a second effort every single time he’s on the puck — he’s relentless. He’d be a pain to play against if I was a defenseman. It’s not easy to come in on this level, Stanley Cup finals, and play at this level. It’s a testament to his work ethic.”

Sanford might get that from his father, who was a cook before he started a furniture repair business. Mike Sanford helped coach Zach, from street hockey sessions to his youth hockey team. The dream of one day hoisting the Stanley Cup was one they shared.

“I know he’s watching,” Sanford said. “He’s definitely pretty psyched.”

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