BOSTON -- Isaiah Thomas had a fourth quarter better than any other player for a team rich in history.
"It doesn't seem real. It's crazy," Thomas said. "The fourth quarter, I've never done that before."
Nobody had -- even for the Celtics. Thomas hit six 3-pointers and converted a three-point play in the fourth quarter and broke the franchise record of 24 points in a quarter set by Larry Bird in 1983 and matched by Todd Day in 1995.
"It just felt like I was out there by myself, working on my game in the gym. Throwing up everything and it was going in. It was a special feeling," said Thomas, who just set his previous career best 10 days before with 44 points at Memphis.
Thomas finished with the fourth-highest total in Celtics history. Bird still holds that mark with 60 points against Atlanta on March 12, 1985.
"It's remarkable," Boston coach Brad Stevens said. "That was an ubelievably willful performance -- and we needed it."
Thomas entered the fourth period with 23 points and scored the first 11 of the quarter for Boston. Al Horford and Jaylen Brown were the only Boston players other than Thomas to score in the final period until Marcus Smart hit two free throws to seal it with 6 seconds left.
Thomas left between the foul shots and received a standing ovation from Boston fans, who chanted "M-V-P!" throughout his run in the fourth quarter. He was 9 for 13 from the field in the fourth period and made all 13 of his free throws in the game.
Horford had 21 points for the Celtics.
Miami opened the fourth on an 11-2 run and led 89-84, but the Heat could not stop Thomas down the stretch. When he wasn't hitting from the outside, Thomas drove the lane for layups.
"I'm not sure there was much we could do more," Miami coach Eric Spoelstra said. "He's one of the very best fourth quarter players in this league for that reason. He doesn't shy from the shot in the moment now. Inside half court he feels like is within his rhythm."
Both teams were coming off losses the night before, but didn't show any fatigue in a tight game Friday.
The Heat opened the fourth quarter with five straight points to start an 11-2 run, including a three-point play by James Johnson and a dunk on a putback by Gerald Green. Richardson stole a pass and took it in for another dunk that gave the Heat an 89-84 lead with 9:27 to play.
Thomas erased the deficit with consecutive 3-pointers, then added another from beyond the arc with 7:14 left to put Boston up 93-89. Thomas wasn't done. He added another 3-pointer to put the Celtics up 101-93 and converted a three-point play with 4:05 left for a 104-98 lead.
The Heat rallied with six straight points to tie it again, but Thomas put Boston right back up with a 1:57 left to make it 106-104 and the Celtics held on.
Heat: Leading scorer Goran Dragic sat out for the second time in three games with back spasms. ... The Heat outrebounded the Celtics 27-18 in the first half. ... Hassan Whiteside had 17 rebounds in each of the first two meetings against Boston this season. He pulled down eight on Friday and scored 11 points.
Celtics: G Avery Bradley was at home with an illness. "He came down with the latest sickness. He came in and got checked out and we sent him home," coach Brad Stevens said. ... Thomas has scored at least 20 points in 18 straight games, the league's longest current streak.
Thomas the first Boston player to reach 50 points since Paul Pierce scored 50 in a double-overtime loss at Cleveland on Feb. 2, 2016. Bird was the last to score 50 in regulation for Boston on Nov. 10, 1989, against Atlanta.
"My teammates just wanted me to make plays. They kept setting screens and calling plays for me," Thomas said. "They were saying, `Go 60!"
The rest of the Celtics celebrated Thomas' accomplishment in the locker room by dousing him with a bucket of ice water, which he said was another first and he hoped would be the last.
"They got me and they got my youngest son. He's over there freezing. I never experienced that," he said. "That wasn't cool."
Heat: Host Detroit on Sunday night, then head out on a six-game Western Conference trip.
Celtics: Host Utah on Tuesday night. The Celtics have won eight straight against the Jazz in Boston.
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