The Miami Heat sustained a gut-wrenching, last-second loss on their third attempt to clinch the Eastern Conference finals, tilting momentum strongly in favor of the Boston Celtics.

However, the Heat, who had to take the play-in route to even reach the playoffs and then upset the top-seeded Milwaukee Bucks to fuel their postseason run, found another gear in their seemingly never-ending fountain of resiliency.

Series MVP Jimmy Butler scored 28 points, Caleb Martin added 26 and the Heat became the second No. 8 seed to reach the NBA Finals by posting a convincing 103-84 victory over the host Celtics in Game 7 on Monday night.

Miami was wobbling after a 104-103 setback on Saturday that featured Boston's Derrick White scoring on a putback just as time expired, evening the best-of-seven series.

Yet even the raucous homecourt atmosphere in Boston couldn't prevent the Heat from reaching the NBA Finals for the second time in four seasons.

"That could puncture a team's spirit," Heat coach Erik Spoelstra said of the Game 6 loss. "But instead, it drove us to more resolve to try to get the job done."

Bam Adebayo recorded 12 points, 10 rebounds and seven assists as Miami finished the job. Teams that held a 3-0 series lead improved to 151-0 all-time in the NBA playoffs.

Jaylen Brown had 19 points and eight rebounds and White scored 18 points for second-seeded Boston, which was just the fourth team in NBA history to force a Game 7 after losing the first three games.

Jayson Tatum had 14 points and 11 rebounds for the Celtics, but he sprained his left ankle during the game's first possession and hobbled throughout the contest.

"That was tough because it kind of impacted me the rest of the night," Tatum said. "It swelled up. It was frustrating that I was a shell of myself. It was tough to move. It was frustrating, especially happening on the first play."

The Heat will face the Western Conference champion Denver Nuggets in the NBA Finals. Game 1 is Thursday in Denver.

"Nobody's satisfied. We haven't done anything," said Butler, who averaged 24.7 points, 7.6 rebounds, 6.1 assists and 2.6 steals in the series. "We don't play just to win the Eastern Conference. We play to win the whole thing."

The New York Knicks (1999 postseason) were the only other No. 8 seed to reach the NBA Finals.

Meanwhile, the Celtics struggled throughout the do-or-die game.

"I thought we played tight," Boston guard Malcolm Brogdon said. "When you play that way, it makes you hesitant on both ends of the ball. I thought Miami played the opposite. I thought they played loose."

The other three teams to force a Game 7 after falling behind 3-0 were the New York Knicks (lost to the Rochester Royals in the 1951 NBA Finals), Denver Nuggets (fell to the Utah Jazz in the 1994 second round) and Portland Trail Blazers (lost to the Dallas Mavericks in the 2003 first round).

Miami shot 48.8 percent from the field, including 14 of 28 from 3-point range. Duncan Robinson and Gabe Vincent added 10 points apiece. Martin made four treys and collected 10 rebounds and Butler also had seven rebounds, six assists and three steals.

The Celtics connected on 39 percent of their shots and were 9 of 42 (21.4 percent) from behind the arc. Boston went 16 of 77 (20.8 percent) from 3-point range over the final two games of the series.

Brown shot just 8-for-23 from the floor, 1-for-9 from long distance and committed eight turnovers on Monday.

"We failed. I failed," Brown said. "We let the whole city down."

Boston trailed 52-41 at halftime, and White kept the Celtics in the contest with 13 third-quarter points.

White put up eight straight points as Boston trimmed a 16-point deficit in half with 8:29 remaining in the third. The Celtics were unable to creep closer than seven, and Martin's short baseline jumper with 1.5 seconds left gave the Heat a 76-66 lead heading into the final quarter.

Miami started the fourth quarter strong with a 3-pointer from Martin and back-to-back baskets by Butler to push the lead to 17. Later, four straight points by Adebayo increased the advantage to 21 with 6:55 remaining. Miami's advantage topped out at 23 points.

"We talked about going to get a tough one on the road and we did just that," Butler said. "But we're not satisfied. We got four more (wins) to get."

Boston missed its first 12 3-point attempts as Miami controlled the first half, leading by as many as 17 points.

--Field Level Media