Boston’s Jayson Tatum blocks a shot by Serge Ibaka of the Raptors during Game 5 of the Eastern Conference semifinals Monday in Lake Buena Vista, Fla. The Celtics can advance to the conference finals with a win in Game 6 on Wednesday. Mark J. Terrill/Associated Press

LAKE BUENA VISTA, Fla. — The game plan from Boston Coach Brad Stevens was simple: Start things off with an increased effort on defense to set an immediate tone.

The Celtics did exactly as he asked – and put the reigning NBA champions on the ropes.

Jaylen Brown scored 27 points and the Celtics never trailed, rolling past the Toronto Raptors 111-89 in Game 5 of the Eastern Conference semifinals Monday night.

Boston leads the series 3-2 and can earn a trip to the East finals on Wednesday.

“We were playing with great purpose,” Stevens said. “You could feel that from the get-go.”

Kemba Walker scored 21 points for the Celtics, whose starters outscored Toronto’s starters by a whopping 93-45. Jayson Tatum scored 18 points, Daniel Theis and Brad Wanamaker each had 15 and Marcus Smart added 12.

“The job’s not finished yet,” Brown said. “We’ve still got a lot of work that needs to be done.”

Fred VanVleet scored 18 for Toronto, which trailed by as many as 30. Norman Powell scored 16 points, while Pascal Siakam and Kyle Lowry managed 10 points apiece. Matt Thomas also had 10 off the bench.

Here’s what Toronto got out of its first 18 possessions: five points on 2-for-15 shooting, a stretch that included everything from missed open layups to airballed contested jumpers, along with five turnovers in the game’s first 8:53.

“I know it sounds crazy .. but I thought our offense was awesome for like the first eight minutes,” Raptors Coach Nick Nurse said. “I thought it was awesome. I don’t think those shots could have been more open.”

They were down 18-5 after all that, and it got worse in a hurry.

A 23-7 run in the second quarter pushed Boston’s lead from 37-25 to 60-32. The Raptors had gone 5 for 6 from the field just before that spurt began and seemed to be getting in a rhythm, but then they missed 11 of their next 13 shots.

“Tonight our defense really set up our offense,” Walker said. “It worked out for us.”

Toronto outscored Boston 28-25 in the third but it didn’t matter; the Celtics’ lead was 87-63 going into the fourth.

“We just didn’t play crisp enough,” Lowry said. “We didn’t look at the rim early enough in the shot clock to attack. But it happens. Now we got to watch the film and make adjustments and play for our lives.”

TIP-INS

CELTICS: Boston is only 17-21 since 2008 when it has a chance to clinch a series. … Boston now has four wins over the Raptors by 15 or more points this season. The rest of the NBA, combined, has one. … The Celtics scored more points in the second quarter (37) than they allowed in the first half (35).

RAPTORS: Marc Gasol was scoreless and didn’t play in the final 20 minutes. VanVleet was the only Toronto starter to appear in the fourth quarter. … Toronto is 9-10 all-time when facing elimination, as will be the case Wednesday. The Raptors went 1-0 last season in win-or-else games, winning Game 7 over Philadelphia in the East semifinals.

YO ELEVEN

Boston led 25-11 after the first quarter. It was the 662nd playoff game in Celtics history and only the fourth time they allowed no more than 11 points in a first quarter; they gave up nine once and 10 twice. Toronto’s 11 points tied for the second-lowest scoring quarter in its team playoff history. The Raptors had nine points in the first quarter at Detroit on April 21, 2002.

REMEMBERING BROCK

Tatum, a St. Louis native, scrawled “R.I.P Lou Brock” on the sole of his sneakers for Monday’s game. Brock, the Baseball Hall of Famer who helped St. Louis win the World Series twice, died Sunday at the age of 81. Tatum never misses an opportunity to represent his hometown; he often wears Cardinals caps, and even donned a pair of sneakers this season inspired by the 2019 Stanley Cup champion St. Louis Blues – who just happened to beat the Boston Bruins for that title.