The Miami Heat forced an already absorbing NBA Finals to a deciding Game Seven after a thrilling 103-100 overtime victory over the San Antonio Spurs on Tuesday tied the series at 3-3.

Miami trailed by 13 points in the third quarter but LeBron James, ditching his trademark headband, turned on the afterburners to spark a Heat fightback.

San Antonio were not about to roll over under the weight of the onslaught, however, rallying to eke out a five-point lead with 28.2 seconds remaining as the championship ribbons were being prepared for them courtside.

James cut the deficit with a vital three-pointer and San Antonio's Kawhi Leonard could only sink one of his two free throws to give Miami a glimmer of hope as they trailed by just three points in the closing seconds.

Ray Allen then took centre stage, finding the basket with his first successful three-point attempt of the night with five seconds remaining to send the game into overtime.

"It's going to be a shot that I remember for a long time. This will go high up in the ranks because of that situation," Allen told reporters.

Then, with the Heat trailing 100-97 in overtime, Allen and James were again decisive, sinking jump shots before San Antonio's Manu Ginobili turned the ball over while driving towards the basket.

Former Boston Celtics guard Allen then completed a memorable performance by keeping his cool to drain two free throws to give Miami a three-point lead they would not relinquish.

"It was by far the best game I've ever been a part of," said four-time MVP James. "I'm blessed to be a part of this."