The Spurs were just seconds away from clinching their fifth title when Miami produced a miraculous turnaround to win 103-100 and send the series into a seventh and deciding game.
Tim Duncan, who scored 30 points and pulled in 17 rebounds but ran out of gas in the final quarter, was at a loss to explain his team's failure to close out the game.
"I don't know what happened in the fourth and in overtime," he said. "Whatever you want to call it. It's just a disappointing loss."
It was a game of unrelenting tension and escalating drama but one San Antonio seemed destined to win.
The Spurs led by 13 points late in the third quarter and by five with 28 seconds to go when the wheels suddenly fell off.
Miami's Ray Allen sunk a three-pointer with five seconds left on the clock to send the game into overtime then drained two free throws in overtime to keep the defending champions in the hunt.
"It's bad, very bad," said San Antonio's Argentine guard Manu Ginobili. "We were a few seconds away from winning the championship and we let it go.
"A couple of rebounds that we didn't catch, a tough three by Ray, a couple of missed free throws, it's a very tough moment."
The lone comfort for the Spurs was that they will at least get another chance with Game Seven scheduled for Thursday in Miami, although history favours the home team in winner-takes-all clashes.
The stakes could not be higher with everything on the line and Tony Parker said the Spurs would have to find a way to quickly get over their disappointment if they wanted to win.
"We have no choice, we have to bounce back. We have to realize we have another great opportunity," said the Frenchman.
"It's going to be another great game. I'm sure with the guys on the team we'll be ready to go on Thursday."
Duncan, who has already won four championships with the Spurs dating back to 1999, said there was no need for panic or major overhauls.
"We'll do what we usually do. We'll watch a little bit of film and make a couple of little tweaks. We put ourselves in a position to win a game," he said.
"We know what we can do. We know that we can win games either here or anywhere else, and we just have to execute for a longer period of time.
"We had a lapse for a couple of minutes here and there."
Ginobili was less optimistic about the challenge facing the Spurs.
"I have no clue how we're going to be reenergized. I'm devastated," he said.
"But we have to. There's no Game Eight afterwards. We're going to have to play our best game, even better than today.
"Shoot better, better defense, less turnovers in my case, but there's no secret recipe for bouncing back."