After an astonishing game-six finish that left the Boston Celtics dumbstruck and the Miami Heat reeling, the two teams must now pick themselves up to do battle again in Monday's NBA playoff series decider.

And neither expects anything less than another epic battle in Boston.

Facing elimination, the Celtics won 104-103 in Miami on Saturday thanks to a heart-stopping buzzer-beater tip-in from Derrick White and pulled level at 3-3 in the best-of-seven Eastern Conference finals.

The Heat were less than a second from booking a place in the NBA Finals against the Denver Nuggets but after leading the series 3-0, Miami will face the real prospect of being the first team in NBA history to lose after taking such a lead.

Momentum and form suggest the Celtics are on course to complete an unprecedented comeback but Heat coach Erik Spoelstra was quick to offer a reminder that his team has a habit of bouncing back from blows.

Miami lost a first play-in game to Atlanta before beating Chicago and entering the playoffs as the eighth seed.

The Heat then defeated the top-seeded Milwaukee Bucks before taking out the New York Knicks and grabbing a surprise three-game lead over Boston.

"When we say we did it the hard way, there were some bone-crushing losses where we did the right things, and then last-second shots, just for the wins against us," said Spoelstra, who has won two NBA championships with the Heat.

"The competitive spirit of this group, we are never to be denied. Even after games like that, we would always come back the next game and find a way to get a win. That's what we have to do right now."

But that will require more than competitive spirit and the right mindset — the Heat need star Jimmy Butler to rediscover his form from earlier in the series.

Over the first three games of the Eastern Conference finals, he was shooting at 46.0% from the field but in the last three, that number has dropped to 36.5% and on Saturday he made only 5-of-21 attempts from the floor.