A No. 7 seed has stunned the defending Eastern Conference champions, four series have already produced overtime, and a rash of star injuries has scrambled championship odds in what coaches and executives are calling one of the most chaotic openings to an NBA postseason in recent memory.

The Indiana Pacers, who sneaked into the playoffs through the play-in tournament, took a 2-1 series lead over the top-seeded Boston Celtics on Wednesday night with a 121-118 overtime win at Gainbridge Fieldhouse, the second straight game in the series decided in the extra period. Indiana guard Tyrese Haliburton finished with 34 points and 12 assists, including a step-back three-pointer with 11 seconds left in overtime that gave the Pacers the lead for good.

“We’re not surprised, even if everyone else is,” Haliburton said in a postgame interview on the floor. “We earned the right to be here. Nobody on this team is treating it like a bonus round.”

Boston coach Joe Mazzulla, whose team finished the regular season 62-20, called the loss “self-inflicted” and pointed to 19 Celtics turnovers and a 22-6 Indiana advantage in fast-break points. Forward Jaylen Brown left the game in the third quarter after re-aggravating a right hamstring injury and is listed as questionable for Game 4 on Friday. The Celtics have not trailed in a playoff series since the 2023 conference finals.

The bigger shock came one night earlier in Miami, where the seventh-seeded Heat completed a sweep of the second-seeded New York Knicks. The 4-0 result is only the second time in two decades that a 2-seed has been swept in the opening round, according to the league office. Heat guard Tyler Herro averaged 31.5 points across the series, including a 41-point performance in Game 3, and veteran forward Jimmy Butler returned from a late-season knee strain to anchor Miami’s defense on Knicks star Jalen Brunson, who was held to 39 percent shooting.

“We’ve been written off for two months,” Butler said after Game 4. “That’s fine. We play better when nobody’s looking.”

The injury picture has dominated coaches’ Zoom calls with reporters. Beyond Brown, Denver center Nikola Jokic is week-to-week with a left wrist sprain suffered in Game 2 against Memphis; Phoenix guard Devin Booker missed Game 3 against the Los Angeles Lakers with a calf strain and is doubtful for Game 4; and Milwaukee forward Giannis Antetokounmpo, who carried the Bucks to a Game 1 win over Atlanta, was listed as questionable for Thursday’s Game 3 with lower-back soreness.

“This is the cost of the regular-season pace we’ve been playing for five years now,” said Dr. Lena Ortiz, a sports-medicine consultant who has worked with several NBA franchises. “Eighty-two games at this intensity, then a hard switch into playoff load — the soft-tissue injuries you’re seeing aren’t random.”

Television ratings have so far rewarded the chaos. ESPN said Tuesday’s Heat-Knicks Game 4, which finished after midnight on the East Coast, averaged 6.2 million viewers, the highest first-round number on the network in three years. TNT’s Pacers-Celtics broadcast Wednesday drew 5.4 million, with the overtime period peaking at 7.1 million.

Out West, the top-seeded Oklahoma City Thunder lead the Sacramento Kings 3-0 and are the only No. 1 or No. 2 seed in either conference yet to drop a game. Thunder guard Shai Gilgeous-Alexander, the regular-season MVP runner-up, has averaged 33 points on 54 percent shooting through three games. Coach Mark Daigneault said the team is “trying not to look past anybody” but acknowledged the favorable health picture has been “a difference-maker.”

Other series remain tight. The Lakers and Suns are knotted 2-2 after Phoenix forced overtime in Game 4 with a buzzer-beating three from rookie wing Marcus Reed; Denver leads Memphis 2-1 despite Jokic’s absence; the Cleveland Cavaliers and Orlando Magic are tied 2-2 in a defense-first series in which no team has scored more than 102 points; and Minnesota leads the Los Angeles Clippers 3-1.

Several owners and league officials, speaking on condition of anonymity because the matter is internal, said Commissioner Adam Silver is monitoring referee performance closely after a wave of complaints from coaches about late-game calls. The league office issued its routine “Last Two Minute” report Tuesday acknowledging two missed calls at the end of Game 3 between Boston and Indiana, including a non-call on a possible offensive foul against Haliburton with 38 seconds left in overtime.

“The reports don’t change outcomes, and we don’t pretend they do,” said Maya Coleman, a spokesperson for the NBA’s basketball operations office. “They’re part of how we hold ourselves accountable.”

Friday’s slate includes Game 4 in Boston, Game 4 in Milwaukee, and a potential elimination game in Sacramento. League executives said second-round matchups, traditionally locked in by the final weekend of April, may not be set until the first week of May given how many series are stretching toward seven games.

For now, the only thing clear about the bracket, one Eastern Conference general manager said, is that “nobody who filled out a bracket in April is going to be right in June.”