BOSTON — Isaiah Thomas scored 29 points and had 12 assists, Kelly Olynyk scored a career-playoff high 26 points and the Boston Celtics used a big fourth quarter to outlast the Washington Wizards 115-105 in Game 7 of the Eastern Conference semifinals Monday night.

Boston advances to the Eastern Conference finals, where it will host the Cleveland Cavaliers in Game 1 on Wednesday.

“What a special opportunity to get a chance to compete against them,” Boston coach Brad Stevens said.

Led by Olynyk, the Celtics got a huge lift from their bench, outscoring their Wizards’ counterparts 48-5. Boston also connected on 11 3-pointers, including 8 of 13 in the second half.

“Everyone gave their best tonight and it turned out well,” Olynyk said. “It was a team effort, hard work. It was a tough seven-game series. Both teams played hard, we outlasted them there. They are a great team. They made big shots all series.”

Bradley Beal led the Wizards, who were playing in their first Game 7 since 1979, with 38 points, including 24 in the second half.

Otto Porter added 20 points. John Wall and Markieff Morris each finished with 18. But Washington also had 15 turnovers, leading to 17 Celtics’ points.

Boston trailed 76-72 late in the third quarter before ending the period on a 13-3 spurt. The run grew to 22-5 in the opening minutes of the fourth as the Celtics’ lead became 94-81.

But Washington quickly responded, scoring seven straight in just 51 seconds to get back within six.

Washington stayed close as Beal continued to score, including a four-point play. But Olynyk then took over. He scored 11 of 13 points for Boston as the Celtics stretched their advantage to 110-100.

“I knew it would come down to the last six minutes of the game,” Washington coach Scott Brooks said. “They had that little run at the start of the fourth quarter.”

Both teams were a little tight in the early going, with neither team scoring a field goal in the first two minutes of action.

The Celtics had the largest lead of the first half at 10-2, but that gap was quickly erased by Washington as Boston started to rack up fouls.

Marcus Smart, Amir Johnson and Olynyk all were whistled for three fouls in the opening 24 minutes, though Smart did inject some life into a second unit, which outscored their Wizards’ counterparts 19-5.

Wall and Beal each were beneficiaries, scoring eight and nine points, respectively, in the second quarter to help the Wizards take a two-point lead into halftime.

The Wizards dropped to 6-4 all-time in Game 7s, and have not reached a conference final since 1979.

The Celtics improved to 22-8 all-time in Game 7s.