BOSTON -- Brad Stevens thought there was a simple explanation for what happened at TD Garden Monday night.

"If Kelly doesn't go nuts and they throw in a couple more threes, we're not talking about the next step," the Celtics coach said after Kelly Olynyk's huge game off the bench led Boston into the Eastern Conference finals with a 115-105 Game 7 victory over the Washington Wizards on Monday night.

Olynyk, appearing on TNT after the game, said he got a call from his coach in the morning, telling him he had to be his "best when your best is needed."

It was needed and Olynyk delivered.

Averaging 8.3 points per game in the playoffs, Olynyk exploded for 14 of his career-playoff-high 26 points in the first 8:34 of the fourth quarter, making five consecutive shots and playing solid defense to help the Celtics put away the Wizards.

The top-seeded Celtics host the defending champion Cavaliers in Game 1 of the Eastern Conference finals on Wednesday and Game 2 on Friday. Second-seeded Cleveland, 8-0 in the playoffs, took the season series 3-1.

"They're tremendous. The defending champs," said Stevens. "I think they're playing better this year than they did last year.

"I've seen them play before. We played them four times. I think they're better than any of the four times they played us."

Asked what it would take to beat the Cavs, Boston's Marcus Smart said, "Perfection."

Boston, keeping the home-court dominance of the season series intact, trailed by five with 5:39 left in the third quarter but outscored the Wizards 21-9 over the remainder of the period. With the Celtics down by three, Isaiah Thomas, who had 29 points and 12 rebounds -- making him the first player to put up those numbers in a Game 7 since 2007 -- hit two 3-pointers and Smart one in a 1:13 span to put the home team ahead to stay.

"Kelly was MVP tonight," Thomas said. "He did it all."

The Celtics then opened the fourth quarter on a 9-2 spurt, but Bradley Beal kept the Wizards alive until Olynyk buried them. Olynyk shot 10 of 14 from the floor and also had five rebounds and four assists, leaving the game to a huge ovation with 1:37 left (he returned briefly in the final seconds).

There were even chants of "Kel-ly" from the boisterous crowd.

"I did hear that, in between the MVP chants for Isaiah," Olynyk said.

Olynyk's heroics helped the Boston bench outscore the Washington reserves 48-5.

"We needed to help the starters, help with Isaiah," he said. "He's been putting us in really good positions, and the type of attention that they were drawing to him, he was getting rid of it (and) putting us in great positions and we had to make plays, and that's what we tried to do."

Olynyk was the victim of a Kelly Oubre Jr. shove in Game 3 that got the Wizards forward suspended for Game 4. Oubre played just six seconds in Game 7, the crowd chanting his name in the final seconds.

Al Horford had 15 points, six rebounds and five assists, Jae Crowder 14 points and five boards, and Smart 13 points, six rebounds and four assists in the win.

Beal led the Wizards, who have not reached the conference finals since 1979, with 38 points, 18 in the final 16:06. Otto Porter scored 20 points and grabbed 10 rebounds, and Markieff Morris contributed 18 points and nine rebounds.

"I hate losing, especially (because) we felt like we were the better team," Beal said. "You credit them because they played their tails off."

Washington guard John Wall had 18 points, 11 assists and seven rebounds, but he shot just 8 of 23 from the floor, 1 of 8 from 3-point range. He missed his last 11 shots, seven of them from behind the arc, after flexing his muscle following a basket with 7:03 left in the third quarter.

"I love him. I love how he plays," Washington coach Scott Brooks said. "That guy is a true winner. ... He's exactly what you need to lead your team. He's exactly what I need for being my point guard.

"The future is bright because he's leading us."

Said Wall: "We (were) one game short tonight, so it's not a satisfying season to me."

All 11 games between these teams in the regular season and postseason -- all physical affairs -- were won by the home team. The Celtics have beaten the Wizards nine straight times at TD Garden dating back to 2014.

The Celtics hit 11 3-pointers for the eighth straight game, leaving them one game shy of matching the all-time streak.

NOTES: Celtics coach Brad Stevens, who borrowed a Bill Belichick line pregame by telling his players to "do your job," on his team's mindset coming in. "Our guys' vibe doesn't ever change. ... That's one of the things I've really appreciated about this team." ... The Celtics played in their NBA-record 30th Game 7, improving to 21-9, 19-4 at home. ... The Wizards were 7-0 in the playoffs when allowing under 110 points, but they fell to 0-6 when giving up more than 110. ... The Celtics are assured of no worse than the fourth pick in Tuesday night's draft lottery as they still reap the benefits of the Kevin Garnett/Paul Pierce trade to Brooklyn. ... The Wizards are 6-4 in Game 7s, 2-4 on the road. ... The Celtics played a Garnett video urging the fans during the first timeout. ... Belichick was spotted in one of the boxes. ... Washington coach Scott Brooks fell to 2-1 coaching in seventh games. He was 2-0 with Oklahoma City, both wins over Memphis.