BOSTON — The Boston Bruins were clinging to a one-goal lead when defenseman Charlie McAvoy caught a skate edge and fell while trying to race Toronto’s Auston Matthews to a loose puck.

The Maple Leafs’ forward — and the top goal-scorer in the NHL this season — had no one left between him and the goal. Bruins goalie Jeremy Swayman came out to try to poke away the puck but didn’t get there in time. Matthews turned and fired at the open net — right off the near post.

“I thought I thought it was 1-1 as soon as he got by Sway,” Bruins coach Jim Montgomery said after Boston pulled away to beat Toronto 5-1 in the opener of the first-round playoff series on Saturday night. “We can’t give Matthews those kind of opportunities. He’s too good a goal-scorer.”

Bruins rookie Jonny Beecher scored on his first career postseason shot on goal to give the Bruins a 1-0 lead less than three minutes in. It was still a one-goal game in the second period when Matthews, who led the NHL with 69 goals this season, caught Swayman away from the crease but banged the puck off the goalpost.

“I just wanted to see if I can get there before him,” Swayman said. “Glad it didn’t go in.”

A minute after Matthews’ miss, Brandon Carlo made it 2-0, then Jake DeBrusk added a pair of power-play goals to open a 4-0 lead. Trent Frederic added an empty-netter to make it 5-1 with just over two minutes left.

David Kampf scored for Toronto, and Ilya Samsonov stopped 23 shots.

“In the second period, we had a couple of chances, really good chances, some of our best chances in the game,” Leafs coach Sheldon Keefe said. “Matthews (had) a chance to even the score. We don’t do that, and ... it ends up in our net. Then the penalties pop up, which allowed them to pull away in the game.

“This time of year, you can’t make those mistakes,” Keefe said. “The team that does ends up on the wrong side of the game.”

Game 2 is Monday night in Boston before the series shifts to Toronto for Games 3 and 4. If the Bruins follow their pattern from the season, it will be Linus Ullmark in net instead of Swayman.

“It’s going to be hard to go away from Sway. He played a terrific game,” Montgomery said. “If we decide to go with Ullmark, we’re comfortable with it, and our team’s comfortable with it. It doesn’t affect us in the room, whoever starts next.”

The Maple Leafs are anxious to end a half-century Stanley Cup drought and snap Canada’s 30-year slump – or at least build on last year, when they won a playoff series for the first time since 2004. But to do it, they’ll have to get past an Original Six nemesis.

Toronto hasn’t beaten the Bruins in the playoffs in six tries since 1959. Boston also swept the regular-season series 4-0.

This was more of the same.

Toronto, which scratched No. 2 scorer William Nylander due to an undisclosed injury, tried to set a physical tone even before the opening faceoff, when Max Domi tried to provoke Bruins captain Brad Marchand. It didn’t work, with Boston opening a 2-0 lead and then taking advantage of the Leafs’ penalties to pull away.

Beecher, who played 17 games in the AHL this season, converted a two-on-one with Jesper Boqvist to give Boston the lead 2:26 into the game.

It stayed that way until about five minutes were gone in the second, when DeBrusk dug the puck out from behind the Toronto net and fed it to Carlo in the slot for the goal. The Leafs picked up back-to-back penalties, and the Bruins scored on both of them before Kampf made it 4-1 and spoiled the shutout early in the third.