NBA roundup: Pacers hold off Celtics in overtime, Cavs rout Lakers in James family's return
Bennedict Mathurin led the Pacers with 30 points and 11 rebounds. Siakam added 29 and Tyrese Haliburton scored 17 points with 12 assists for Indiana, who led by as many as 24 early in the third quarter.
Boston outscored Indiana 38-24 in the fourth, twice cutting the deficit to three before Jayson Tatum drained a step-back three-pointer with 14.4 seconds remaining in regulation.
Boston took their first lead since the first quarter on Derrick White's driving layup that made it 132-130 with 39.7 seconds left.
But Siakam had the last word, making a basket to tie it then rising for a three-pointer over Haliburton with 7.3 seconds to play.
"It's big, it's big," Siakam said. "We got a couple of games where we lost by a couple of turnovers, rebounds. So we wanted to get this one."
Tatum scored 37 points, Brown added 25 and White scored 23 for Boston, but they couldn't pull off the miracle.
"We let them come back, but I thought we fought," Siakam said. "That's what we've got to do. We've got to be a team that fights to the end."
Boston dropped to 4-1 as Cleveland improved to 5-0 for just the third time in franchise history.
Indiana's victory in a rematch of last season's Eastern Conference finals left the Cleveland Cavaliers as the East's only unbeaten team.
The Cavs gave the Los Angeles Lakers a rough ride, despite a warm reception for Bronny James in Cleveland, where the hosts dominated in a 134-110 victory to improve to 5-0.
Wednesday's game was billed as a homecoming for LeBron and Bronny James - who hadn't played since he was subbed in to play alongside his superstar father in the Lakers' season-opening victory over the Minnesota Timberwolves.
That made them the first father-son duo to play alongside each other in an NBA game.
This time around, LeBron James had departed for good and the game was out of hand when Bronny James was subbed in late in the fourth quarter - to cheers from Cleveland fans who remember him as a toddler attending Cavs games when his father starred for his hometown team.
Turnovers cost Lakers
Although the 20-year-old drafted by the Lakers in June is expected to be headed to the developmental G League soon, he did hit a mid-range jump shot for his first NBA points.
"To see him get his first NBA basket in this arena where he grew up not too far from here - it's an unbelievable moment," LeBron James said.
Nevertheless, it will be little consolation for the Lakers.
The Cavs made 17 three-pointers to the Lakers' six and parlayed 21 Lakers turnovers into 31 points.
Cleveland took control with a red-hot first quarter, connecting on 77.3 per cent of their shots from the field.
They led by 19 at half-time and turnovers doomed the Lakers' efforts to make significant inroads in the third.
Evan Mobley scored 25 points and Mitchell added 24 as six Cavs players scored in double figures.
LeBron James led the Lakers with 26 points. Anthony Davis added 22 with 13 rebounds but Los Angeles dropped their second straight.
The Oklahoma City Thunder remained unbeaten in the West, improving to 4-0 with a 105-93 victory over the San Antonio Spurs.
Lu Dort scored 20 points and Chet Holmgren added 19 for Oklahoma City as San Antonio star Victor Wembanyama was held to six points on one-for-five shooting.
Wembanyama added eight rebounds and three blocks, but it was Holmgren who came out on top in their match-up of second-year big men.
Elsewhere, Karl-Anthony Towns erupted for 44 points and 13 rebounds to propel the New York Knicks to a 116-107 victory over the Heat in Miami.
Jaden Ivey scored 23 points and Cade Cunningham added 22 to lead the Detroit Pistons to their first win, a 105-95 triumph in Philadelphia where the 76ers continued to struggle with stars Joel Embiid and Paul George sidelined by injury.