Raef LaFrentz added 15 points and 14 rebounds to help the Celtics (26-25) move above .500 for the first time since Nov. 12.
"It feels good. I think we've finally gotten over the hump," said Pierce. "Our goal was reaching .500 and getting over it and we've reached that."
Boston outscored New York 27-11 in the third quarter en route to its fifth victory in six games and its fourth straight over the Knicks.
New York dropped its 14th game in 16 tries to fall to 3-18 in 2005 and 19-31 overall. Stephon Marbury scored 31 points, his fifth straight game with 30 or more, but it wasn't enough as the Knicks fell 6½ games behind Boston in the Atlantic.
"It's always disappointing when you lose, but when you lose a division game its like a double-whammy," Knicks coach Herb Williams said. "We've got to make that up somewhere."
Up 57-51 at halftime, Boston outscored New York 19-5 to start the second half. LaFrentz had seven points, seven rebounds and two blocks in the quarter, and Mark Blount's layup with 4:40 left in the period put the Celtics ahead 76-56.
New York closed to 95-82 with 5:01 left, but LaFrentz buried a 3-pointer on Boston's ensuing possession to put an exclamation point on one of his finest games of the season.
"Guys are coming together. Guys are sharing the ball," Davis said. "All the bad habits are starting to creep out the door."
Boston held the Knicks to just two field goals, both by Kurt Thomas, in the opening 8:54 of the second half as the Celtics put together their best defensive quarter of the season. But Williams placed the blame on New York's sloppy play -- especially its poor rotations on defense -- and not on Boston's pressure.
Davis and LaFrentz each buried open 3-pointers during the Celtics' key stretch, and Tony Allen added another jumper from just inside the arc as Boston took advantage of yet another inconsistent defensive effort by the Knicks.
"We just got lax," Williams said. "We were leaving guys open at the 3-point line and they were knocking them down."
Boston, which won its fourth straight home game, hasn't been above .500 since beating Charlotte 91-74 in November to improve to 3-2.
Counting the Red Sox's four-game comeback against the New York Yankees in the American League Championship Series, the Patriots' regular-season sweep of the Jets and the Celtics' current four-game run over the Knicks, Boston pro sports teams have won 10 straight over New York teams. ... The last time the Celtics were over .500 this late in the year was on April 16, 2003, when they finished the season 44-38.