Boston enters the break with an Eastern Conference-best 44-11 record, including victories in 15 of the last 17 games. The Celtics have won nine straight on the road.
Rivers was ejected in the third quarter after getting two technicals for arguing calls.
The Celtics opened the fourth with a 7-4 run to trim what had been a 15-point deficit to 79-76 after a jumper by Pierce. Rajon Rondo's lay-in with 8:01 left brought Boston within 82-79. With 5:37 remaining and a well-rested Kevin Garnett, Ray Allen, and Kendrick Perkins on the bench, Pierce gave the Celtics their first lead of the game, 85-84, on a 3-point play with 5:37 remaining. Garnett returned with 4:44 remaining, Allen with 4:02 left.
Dallas forward Josh Howard (17 points) fouled out with 3:47 left after fouling Pierce on a jumper. Pierce nailed both free throws to give Boston an 87-84 lead. Antoine Wright's lay-in sliced Dallas's deficit to 87-86 with 2:39 remaining.
The rest of the game became a battle between Pierce and Mavericks star Dirk Nowitzki, who scored a game-high 37 points.
"It was just about getting stops and getting back in the game," Pierce said. "The fourth quarter, I started getting a rhythm. Guys were going to me at the top of the key. We had a play that was working and just milked it. We spread the floor and I thought I had the advantage every time I got into my sweet spot and took shots. I started to feel good."
Rondo's 20-foot jumper pushed Boston ahead, 89-86, with 2:14 left. But Nowitzki's 3-pointer with 1:58 remaining knotted the score at 89. Pierce, however, nailed two free throws with 1:42 left to give the Celtics a 91-89 advantage. Nowitzki hit a 12-foot turnaround jumper with 1:28 left, but Pierce responded with an 11-foot turnaround jumper with 1:12 remaining to give Boston a 93-91 lead.
Nowitzki couldn't return Pierce's serve as he was called for an offensive foul with 1:01 left. Pierce came back with a 14-foot step-back jumper with 38.5 seconds left to give Boston a 95-91 lead. With Pierce barking at the Dallas bench, the Mavericks called time out.
Nowitzki made 1 of 2 free throws with 26.6 seconds left to trim Dallas's deficit to 95-92. But Allen nailed two free throws with 19.2 seconds remaining to seal the win.
