Morris was one of six players in double figures for the Suns, who have won back-to-back games coming out of the All-Star break.
Rajon Rando led the sluggish Celtics, who lost for the third time in their last four games. Rondo finished with a double-double of 18 points and 10 assists. Brandon Bass also scored 18 and Jeff Green added 17.
Boston rallied in the fourth quarter and cut the Suns' lead to 88-86 with 3:59 to play on a 3-point bucket by Kelly Olynyk, but the Celtics immediately hit a cold streak and did not score again until Gerald Wallace sank a pair of free throws with 1:39 to play to make in 92-88.