MEMPHIS, Tenn. -- Isaiah Thomas, in the words of Boston Celtics teammate Avery Bradley, "did what Isaiah does -- put the ball in the basket."

Thomas did it so often he scored a career-high 44 points to lead the Celtics to a 112-109 victory in overtime against the Memphis Grizzlies on Tuesday night at FedExForum. He scored 36 of his points after halftime.

"I wasn't trying to get 40," Thomas said. "I just knew I had to get a little more aggressive."

Said Boston center Al Horford (17 points and 14 rebounds): "He just carried us. He takes a lot of pressure off our team. He just makes it look so easy."

The Celtics (16-12) led by as many as five points late in overtime They erased a nine-point deficit at the start of the fourth quarter and Thomas hit two free throws with 27 seconds left in regulation to tie the game 97-97.

Memphis center Marc Gasol (24 points, seven rebounds, six assists) missed a turnaround jumper with 1.9 seconds left. Horford had a chance to win the game in regulation but his short shot went in just after time expired.

At halftime, Memphis led 45-31 and had a 32-19 rebounding edge, 12-4 on the offensive glass.

"Their offensive rebounding was killing us," said Celtics forward Jae Crowder (13 points).

Memphis coach David Fizdale went into halftime happy his team had held Boston to 31 points and then watched the Celtics go for 31 points in the third quarter and 35 in the fourth.

"So basically, we stayed in the locker room at halftime," Fizdale said. "Our defense totally failed us."

Thomas scored the first nine points of the fourth quarter for the Celtics and assisted on the next basket as Boston cut into the Memphis lead. The Celtics gained their first lead of the second half 86-85 on a layup from guard Avery Bradley (16 points) with 4:17 left.

Thomas finished 10 of 16 from the floor, 7 of 10 from 3-point range, and made a career-best 17 of 17 at the foul line. Thomas also had six assists, three rebounds, two steals and a block.

"I mean, he had his hands on the ball every possession," said guard Troy Daniels, who had 24 points off the bench for Memphis.

Mike Conley finished with 19 points and eight assists and the Grizzlies (18-12) lost their third straight after leading by as many as 17 points in the second half. They were up 63-46 in the third quarter before the Celtics went on a 14-1 run to trim the lead to four points. Memphis carried a 71-62 lead into the fourth quarter.

Boston shot 40.7 percent from the floor and was 12 of 37 from 3-point range. The Grizzlies shot 40.4 percent from the field and were 12 of 30 from distance.

Memphis power forward Zach Randolph (nine points) was assessed a flagrant 2 foul in the overtime and ejected.

Gasol said wasn't difficult to explain how the game got away from Memphis.

"Too many easy baskets," he said. "They got to the line a lot (34 of 38). They got control of tempo of the game, and obviously we lost the tempo of the game."

NOTES: Memphis fell to 4-1 in overtime this season. ... After Tuesday's game, nine of the Grizzlies' next 12 games are on the road. Memphis is 7-5 away from FedExForum. ... Memphis F JaMychal Green recorded his fifth double-double of the season in Sunday's loss to Utah. He increased his rebounds per-game average from 4.8 last season to 8.0 this season. ... Tuesday marked Memphis PG Mike Conley's third game back in action after suffering lower-back fractures Nov. 28 vs. Charlotte. He's playing without pain medication. "Tough cat," Grizzlies coach David Fizdale said. ... The Celtics came into Tuesday's game with a 3-7 record against Western Conference teams. ... Grizzlies GM Chris Wallace held the same position in Boston from 1997-2007.