DeMar DeRozan scored 22 points on Friday and the Raptors defeated the Celtics, 84-79.
The Celtics had won four straight entering Friday and could have clinched a playoff berth with a victory and a Milwaukee loss. They still hold a three- game lead in the Atlantic Division by virtue of a 76ers loss to the Nets on Friday.
Paul Pierce had 18 points and eight rebounds in the setback, but missed a potential tying three-pointer in the closing seconds.
"We didn't have it in the second half," said Celtics coach Doc Rivers. "It's kind of my fault. I contributed to it by giving our starters too much rest and kind of threw off their rhythm."
The Raptors, who had lost four in a row and were playing without Andrea Bargnani and Jose Calderon, got 17 points from Linas Kleiza and 12 from Alan Anderson.
Toronto led by as many as 15 in the fourth, but Boston made a late push, cutting its deficit to 76-75 on Pierce's three-point play with 38.8 seconds remaining.
DeRozan then drew a foul on a baseline drive and made both free throws for a 78-75 Raptors edge. Pierce answered with a dunk, but DeRozan made two more from the stripe and Pierce's three-point attempt with 13.2 seconds left was off the mark.
Kleiza made four free throws the rest of the way to seal the win.
"I was just trying to be aggressive, trying to go to the basket," said DeRozan. "That's what it's all about, making it to the basket and making free throws."
The Celtics led by as many as 13 in the opening half, but the Raptors cut their deficit to 36-30 by the break and used an 8-2 run to tie it at 38 early in the third.
They continued to surge ahead, grabbing a double-digit advantage, 54-43, on DeRozan's three-pointer before taking a 57-47 lead into the fourth.
Toronto split the season-series with Boston for the first time since the 2001-02 campaign...Bargnani will miss the remainder of the season with a strained left calf. Calderon was out with a swollen right eye...The Celtics were without shooting guard Ray Allen (sore right ankle).