PHOENIX -- Isaiah Thomas was making his return to Phoenix, but it was Avery Bradley who had the more adventurous trip to the Valley of the Sun.

Thomas scored a game-high 28 points in his first game in Phoenix since being traded a year ago and Bradley added 15 as the Boston Celtics, after squandering a 21-point lead, held on to beat the Suns 102-99 Saturday night.

After being hospitalized with an undisclosed illness, Bradley skipped the team flight to Phoenix. Instead, he flew out Saturday morning and arrived in town a few hours before tipoff.

"I was tired," Bradley said. "The first half, I actually felt good. I was just trying to fight through it. I started getting a little tired in the second half but I just want to play as hard as I can."

Bradley's extra effort was needed on a night when the Celtics twice let double-digit leads -- first 21 points and later 10 -- slip through their fingers.

"They can come back in a hurry," said Celtics coach Brad Stevens. "They're going to keep firing. I thought we made defensive plays and obviously made enough free throws to win the game," Stevens said. "But offensively, I thought we didn't play with much poise in the third quarter and that's why they got back in the game."

Devin Booker capped an 8-2 run with a step-back jump shot that pulled Phoenix within 98-97 with 1:16 remaining. Booker had a chance to give the Suns the lead on their next possession, but his 20-footer bounced off the back of the rim.

Phoenix got another opportunity when Thomas missed a 3-point attempt from the right elbow but Brandon Knight threw up a 25-foot 3-pointer that fell well short of the rim.

Thomas handed the Suns a final chance when he missed one of two free throws with 13.9 left, but Evan Turner blocked Booker's driving attempt with 3.9 seconds to go to secure the win.

"I was just trying to get to the line and get contact," Booker said. "They were being aggressive with me. But I found out now that in this league, ate in ball games likes that, a ref is not going to call a foul so you just have to shoot your shot and can't be looking for any time of foul."

Thomas, sent to Boston by the Suns just ahead of the trade deadline in 2015, was only 8-for-20 from the floor in his first game back in the arena where he played for four often tumultuous months, but was 9-of-10 from the floor.

Turner scored 17 and Kelly Olynyk added 16 for the Celtics (43-30), who won their fourth straight and moved a half-game ahead of Miami for fourth place in the Eastern Conference.

Booker, after scoring just two points in the first half, finished with 21 and Knight added 19 for the Suns (20-53), who have lost two straight and three of four.

"I can't say everything I said (at halftime)," said Suns coach Earl Watson. "It'd be a lot of beep, beep, beep, beep. More beeps. You have to stop turning the ball over, play with purpose and be accountable. And then I said I love you, but it's tough love I guess."

The Celtics led by as many as 21 in the first half and held a 64-44 lead with 10:12 left in the third quarter, only to watch the Suns come roaring back.

PJ Tucker and Knight each converted 3-point plays and Knight scored nine points total as Phoenix outscored Boston 29-10 over the next seven-plus minutes. Booker's dunk off a pass from Tucker pulled the Suns within 74-73 with 2:43 to go in the quarter.

Thomas steadied the Celtics, scoring seven straight points to close the quarter and help Boston take an 81-75 lead after three.

"They started hitting tough shots and then those tough shots turned into easy shots," said Celtics guard Marcus Smart, who finished with nine points and eight rebounds.

Boston pushed the lead back to 10, 86-76, on an Olynyk lay-in off an offensive rebound with 10:56 remaining before the Suns mounted their late fourth-quarter comeback.

NOTES: The Suns are 30-70 since trading Isaiah Thomas and fellow G Goran Dragic at the 2015 trade deadline. ... Celtics F Jae Crowder, who has missed the last six games with a high ankle sprain, did not play Saturday and also will miss Monday's game against the Los Angeles Clippers. ... Suns F Jon Leuer was scratched with an upset stomach. G/F John Jenkins, a starter Friday night at guard, took Leuer's starting spot as the small forward. ... Suns G Ronnie Price and G Brandon Knight, also battling stomach ailments, dressed. ... Suns C Tyson Chandler missed his third straight game with back spasms. ... The Suns were playing the back half of their last set of back-to-back games. ... The 1975-76 "Sunderella Suns" were honored at halftime of the game. That Suns squad reached the NBA Finals before losing in six games to the Celtics, one of two Phoenix teams to make the championship series in the franchise's 47 seasons.