Jason Terry scored Boston's last nine points and the Celtics weathered a strong comeback by the New York Knicks, avoiding a sweep with a 97-90 overtime win on Sunday.
Paul Pierce led the Celtics with 29 points as they forced a fifth game in the series, but it was Terry who provided the finishing touches.
In Game 3, he had been elbowed by J.R. Smith, and the NBA suspended the Knicks guard for Sunday's game.
New York had tied the game 84-84 after trailing by 20 points early in the third quarter. It was 88-all before the Celtics regained control and took the lead for good on a 3-pointer by Terry. Carmelo Anthony, who led the Knicks with 36 points, hit a short jumper, but Terry connected on a 15-footer with 50 seconds remaining for a 93-90 lead.
After Anthony missed a 3-pointer with 21 seconds to go, Terry was fouled and sank both free throws. He added a layup to close out the game.
But the Celtics still have a huge deficit in trying to become the first team to win after trailing a series 3-0.
Game 5 is Wednesday night in New York, where the Knicks can win their first playoff series in 13 years.
Jeff Green scored 26 points for the Celtics and Terry finished with 18.
The Celtics showed renewed energy early after being held below 80 points in each of the first three games. They led 54-35 at halftime and 59-39 three minutes into the third quarter before their recent second-half woes returned. In previous first halves, they scored just 25 points in Game 1 and 23 in Game 2. On Sunday, they were outscored 30-14 in the third quarter and led just 68-65 heading into the fourth.
Boston held a 65-51 lead when Anthony went to the bench with 3:35 remaining. The Knicks outscored the Celtics 14-3 the rest of the way behind 11 points from Felton and a 3-pointer from Iman Shumpert. Felton finished with 16 points in the quarter and 27 in the game.
The Knicks played the first half as if they had taken shooting lessons from the Celtics. New York hit just 28.9 percent of its shots (11 for 38) after Boston made only 39.5 percent of its attempts in the first three games.
The Celtics found their range from the start and connected on 51.3 percent (20 for 39).
Notes: The Knicks were 19-2 in their previous 21 games. The Celtics were 5-14 in their previous 19. ... For the Celtics, Garnett, Green and Brandon Bass each had four fouls five minutes into the third quarter. Bass committed his fifth with 5:10 left in the period and fouled out with 4:27 to go in the games. ... Anthony committed his fourth with 4:08 remaining in the third.