No one could have faulted Boston Celtics guard Isaiah Thomas had he chosen to sit this one out.

Thomas, playing in tough circumstances a day after his sister was killed in a single-car accident in Washington state, seemed to give his team an emotional boost on top of scoring a game-high 33 points. He hit clutch shots and embraced teammates, but it wasn’t enough as the eighth-seeded Chicago Bulls stunned the top-seeded Celtics 106-102 and took Game 1 of their first-round series.

“We’re never going to make excuses, but this is hard,” Celtics center Al Horford told reporters. “This was difficult, and I felt like our guys really dealt with it best. We knew (Thomas) was hurting. … We have a lot of respect for him that he was able to come out.”

The Bulls led 103-94 with 47 seconds left before the Celtics staged a late rally on the back of two three-pointers and a double-clutch drive and finish from Thomas. Jimmy Butler eventually sealed the game with three late free throws.

Butler finished with 30 points and nine rebounds, and reserve Bobby Portis chipped in 19 points off the bench. More than that though, the Bulls played hard-nose defense in the fourth quarter — aside from the late near-meltdown — to seize homecourt advantage. In an area where many felt the Celtics could be exploited, the Bulls pounded Boston on the glass 53-36.

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“We were so locked in all week,” Butler told reporters. “We knew their stuff, just like they knew ours. But we executed extremely well. We haven’t done that all season, but this is the right time to do it.”

Rajon Rondo said the Bulls wanted to be aggressive. “I told the guys coming out it was going to be physical and we wanted to throw the first punch.”

Game 2 is Tuesday night in Boston.

Thomas received a warm ovation from the Boston crowd during pregame and when the teams took the court for the opening tip-off. Thomas started fast, scoring nine of the Celtics’ first 15 points. He slowed slightly after that and finished the first half with 15 points and three assists.

One of his three assists in the first half came on a dish to Avery Bradley, who buried a three-pointer from the perimeter. Seconds later, after the Celtics forced a turnover, Thomas whipped a through-the-legs pass to Bradley again, who kicked it back to Thomas for a deep three-pointer. The Bulls called a timeout and Bradley immediately found Thomas to embrace him.

“He’s a hell of a player,” Butler said. “It just goes to show the type of player and man he is to go out there and battle through what he was going through for his organization and team.”

Contributing: Associated Press.