OKLAHOMA CITY -- The first time Boston Celtics guard Marcus Smart took on the Oklahoma City Thunder, he set a career high. He decided to double down on that performance in his second trip to Oklahoma City.

Smart scored a career-high 26 points, leading the Celtics to a 100-85 victory over the Thunder on Sunday at the Chesapeake Energy Arena.

Taking on the challenge of going toe to toe with Thunder guard Russell Westbrook, Smart shot 9-for-14 from the field and hauled in eight rebounds as the two intense guards battled for four quarters. Despite his youth, Smart said he wasn't going to back down from the veteran.

"That's the type of guy that Russ is," Smart said. "He loves challenges. He is going to try to do his best every time. And vice versa with me. You put two guys like that going against each other, obviously you're going to knock heads.

"Not intimated at all. You can't be. Russ knows that and understands it. I know it. I love those type of challenges."

However, Westbrook isn't ready to put Smart on his level just yet.

"He had a good game," Westbrook said of Smart. "Eighty-two games, I do this. Don't get it twisted. We'll see him again."

With guard Avery Bradley coming off the bench, the Celtics reserves torched Oklahoma City in the fourth quarter. Boston built an 11-point advantage and had control of the game midway through the period.

Oklahoma City brought its starters back in, but they lacked the energy to contend with the scrappy Boston squad. They were beaten to loose balls and rebounds and were just out-hustled.

The Thunder never got closer than 12 points the rest of the way.

Westbrook had a game-high 27 points on 5-of-20 shooting. He shot 2-for-10 from behind the 3-point arc but was 15-for-17 from the free-throw line and was held to season-low five assists.

Forward Serge Ibaka was the only other Thunder player to score in double figures, finishing with 16 points and 10 rebounds. Center Enes Kanter was held to eight points and two rebounds.

"I got no answers for that," Kanter said of his team's offensive performance.

The Thunder dropped to 6-4 overall, 1-1 without injured forward Kevin Durant.

Boston guard Isaiah Thomas scored 20 points to go along with eight assists. Center Jared Sullinger owned the glass, as he pulled down 15 boards. As a team, the Celtics outrebounded the Thunder 47-34.

"We need to pick our intensity up physically," Westbrook said. "Just wasn't ready."

Back in the state where he played his college ball, Smart started the night as if he were still playing for Oklahoma State. He shot 4-for-6 in the first quarter to knock in nine quick points. He would have hit double digits if he hadn't missed three free throws.

"I think that he likes this gym," Boston coach Brad Stevens said. "This is the gym he always shoots it well in, right? I think his offense has really improved."

While Smart was attacking the basket, Westbrook was doing his damage from the perimeter. He attempted five 3-pointers in the first eight minutes of the game. He entered the game averaging only five 3-pointers a game on the season.

Oklahoma City was able to build a 10-point lead with less than a minute left in the first half, but the Celtics cut the deficit down to 52-45 at halftime.

The Celtics stormed all the way back in the third quarter to tie the game at 72-72. Smart continued his rampage and seemed to score at will, but it was forward Jae Crowder's energy and defense that allowed the Celtics to storm back and only trail by two heading into the fourth quarter.

"We got back into the game at the end of the third," Stevens said. "That was a big key. It gave us a little bit of momentum heading into the fourth. We were really just active defensively."

NOTES: Oklahoma City was averaging around 20 turnovers through its first four games, but in their last five games before Sunday, the number dropped to 15. The Thunder gave the ball away 18 times against Boston. ... After missing two games with a calf strain, Boston G Avery Bradley was back in the lineup. He scored 14 points in 26 minutes. ... Thunder G Anthony Morrow got his second start of the season, but he finished scoreless in 15 minutes. ... Thunder F Kevin Durant on his strained hamstring: "Feel a lot better. Feels way better, actually. You can re-injure (hamstrings) pretty easily if you try and play through it, or as we say now, 'Be tough.' I call that being dumb."