CLEVELAND — For all that LeBron James has accomplished in his glowing career, only once did a season start this well for his team — and it ended with a championship over Kevin Durant.

James scored a season-high 30 points to go with his 12 assists Thursday, including 20 points in the third quarter, in the Cleveland Cavaliers’ 128-122 victory against the shorthanded Boston Celtics.

Kevin Love scored a season-high 26 points and Kyrie Irving scored 23 for the Cavs, who improved to 5-0 for the first time since the 1976-77 season.

It’s only the second time James has been 5-0 in his career. The other time, to start the 2011-12 season, ended with the Miami Heat’s sweep against Durant’s Oklahoma City Thunder for James’ first NBA championship. And with a road game at the Philadelphia 76ers up next on Saturday, James could easily be looking at 6-0 for the first time in his career.

“We want to continue to get better,” said James, who is two points shy of Hakeem Olajuwon (26,946) for 10th place on the NBA’s all-time scoring list. “When we get up on teams we’ve got to figure out how to continue to just push the lead up or just keep it where it is and not allow teams to get back into the game.”

Isaiah Thomas scored a season-high 30 points and Avery Bradley had 26 points and 10 rebounds for the Celtics, who were wounded and playing on the second night of a back-to-back.

Big free agent pickup Al Horford is in the league’s concussion protocol after taking a blow to the head in practice this week, while Jae Crowder sprained his ankle in Wednesday’s win at Chicago.

The Cavs raced out to a big first-half lead Thursday while James was content to facilitate. He had five assists in the first quarter and eight in the half while the Cavs built an 18-point lead. He kicked into attack mode in the third quarter to keep the lead in double figures following a Boston surge.

The Celtics are one of the teams expected to push the Cavs in the Eastern Conference this season, but they were simply outmanned Thursday.

“This is the best I’ve seen the Cavs play in November,” Celtics coach Brad Stevens said. “Everybody is trying to figure themselves out to be the best version of themselves, but they’re way ahead of where they were the last two years, in my opinion. And (James) looks great. I mean, they’re really good.”

The Celtics cut a 20-point deficit down to four in the fourth quarter. They were within 115-109 on a driving layup by Thomas with 3:15 left, but didn’t get any closer.

“We’re always going to be in games because we’re one of the hardest working teams in the NBA. That’s half the battle,” Thomas said. “It’s very disappointing we lost, but there are no excuses. We played a game we felt like we could’ve won. We didn’t play that well and only lost by six.”

Tristan Thompson had 15 points and 14 rebounds for the Cavs, including 10 rebounds in the first quarter. This marks the Cavs’ best start since the 1976-77 season.

“We’re three years in with one another. We know what to expect,” Irving said. “We have something that we’re building here that’s very special. And the trust and responsibility and everything that we depend on from one another is already there. It’s not like we had to come in training camp and had to figure things out right away. For us, it’s about staying healthy and maintaining our edge on other teams.”

NOTES: F LeBron James attended the Cleveland Indians’ Game 7 loss in the World Series to the Chicago Cubs. James is 3-4 in championship games. “You never get past the pain. I still think about and dream about or have nightmares about losing in the (NBA) Finals,” he said. … The Cavs and Celtics could meet in the playoffs, but James isn’t looking that far ahead. “It’s early November,” James said. “So we’re still trying to implement our system.” … C Tyler Zeller registered his first double-double of the season with 11 points and 10 rebounds. … Celtics coach Brad Stevens’ wife, Tracy, is a Cleveland native. She flew on the team plane to Cleveland late Wednesday during the World Series. “We lifted off with one out in the bottom of the 10th inning, and so we lost internet connection a little bit,” Steven said. “And so that was an interesting time to be in the air. We were in suspense for sure.”