The Clippers grinded defensively through the fourth quarter, allowing Toronto only 10 points in the final period as the Raptors missed six of their first seven shots while Lou Williams came off the LA bench for a game-high 21 points, AFP reported.
"Just compete on the defensive end, contest their shots and get back on transition," Leonard said as the secret for the Clippers' late success.
Leonard had 12 points, 11 rebounds and nine assists but shot only 2-of-11 from the floor and 0-of-4 from three-point range.
"That's what a championship team is about," Leonard said.
"When your star is not making shots, they step up and pull it out. That's what it's all about."
Leonard was the NBA Finals Most Valuable Player for Toronto in June when he sparked the Raptors over Golden State in the best-of-seven championship series, but he departed for his hometown Clippers in July.
"It was fun to compete against them," Leonard said, calling the experience emotional but keeping his trademark composure.
"We had a fun ride last year. On and off the court we had fun."
Celtics extend streak
Pascal Siakam led the Raptors with 16 points and 10 rebounds while shooting 6-of-17 from the floor.
The Raptors, without injured Kyle Lowry, lost forward OG Anunoby to injury after only two minutes on the court.
Boston guard Kemba Walker scored 29 points before leaving late in the game with an injury, leading the host Celtics over Dallas 116-106.
Celtics coach Brad Stevens called the setback "a little whiplash" and did not expect it was serious.
Jaylen Brown added 25 points and 11 rebounds, his second consecutive double-double, on a night when Jayson Tatum went 1-for-18 from the floor and Luka Doncic led Dallas with 34 points.
Harden best since '67
NBA scoring leader James Harden netted 39 points and Russell Westbrook added 26 to lead the Houston Rockets to a 122-116 victory at New Orleans.
With 373 points in 10 games, Harden becomes the first player to score so many so quickly into an NBA season since Rick Barry had 381 in 1967. The only player ever with more was 1960s icon Wilt Chamberlain.
Clint Capela had 11 points and 20 rebounds for the Rockets while JJ Redick led the Pelicans with 24 points.