Chauncey Billups' scored 18 points, seven assists and Carmelo Anthony notched another double-double (18 points, 13 rebounds) as the Nuggs' knocked off the C's, 94-85.
Nene also had 14 points, seven rebounds, and five steals for the 5-4 Nuggets.
Ray Allen led the Celtics' with 26 points, Kevin Garnett had 16 points, nine rebounds, and two blocks, and Paul Pierce chipped in 19-7-4.
The Celtics opened the game on an 8-0 run but had eight turnovers in the first quarter, which resuscitated the Nuggets as Denver went on a 13-5 run to knot the game at 13-13.
However, the C's managed to hang onto the lead thanks to the hot shooting Ray Allen. Allen went six-of-seven in the first, and scored 18 points to lead Boston.
Chauncey Billups went four-of-five from the field and scored nine points in the opening 12 minutes to keep Denver in the game.
The second quarter proved to be the deciding portion of the contest.
The Nuggets went on a 19-2 run that seized the Celtics lead and made it 48-36, distancing themselves from the C's. Boston helped Denver by converting on only one field goal with 6:21 left in the second.
Hoping to stop the bleeding, the Celtics went on a 6-0 run to close out the half, as Denver lead 48-42 at intermission.
Nene had 10 points, Billups had nine points and four assists, and Carmelo had eight points and eight rebounds.
Ray Allen had 18 points (0 in the second), Garnett had nine points and four boards, and Pierce contributed seven points and four rebounds for the Celtics at halftime.
Denver out-rebounded Boston 24-17 at halftime, including 11 offensive rebounds to Boston's seven.
The Nuggets continued their assault on the Celtics by opening the third on a 9-0 run that pushed their lead to 59-44. However, Boston countered the Nuggets' run by executing a 21-5 tear that handed them the lead 65-64 going into the fourth via an Eddy House three pointer.
In the fourth though, Denver regained the lead thanks to the antics of J.R. Smith, Anthony Carter, and Linas Kleiza. Smith scored six crucial points for the Nuggets featuring a three-point play that tied it at 67 and a three pointer over Ray Allen that knotted the score at 70.
As for Kleiza, Linas knifed around Ray Allen for a transition lay-up that gave Denver its first lead since the third (72-70) and then converted on another three-point play moments later that pushed the lead to 75-70 Denver. Moments later, J.R. Smith swished another bomb, making it 78-70. And what about Anthony Carter? He was the ring leader doling out
After the two teams traded baskets, the game was once again deadlocked at 84. However, the Nuggets went on a late 8-0 run to close out the game and give Boston their first home loss this season (5-1).
Kenyon Martin made a driving lay-up, Nene stripped Kendrick Perkins and Billups' gobbled up the ball and drove the ball to the basket. Chauncey was then fouled by House and completed a three-point play that pushed the lead to 89-84.
A series later, Carmelo drained a three pointer sealing Denver's win.
A very impressive win for the 5-4 Denver Nuggets, as they defeated the defending World Champs.