The Spurs responded with seven straight points without him to get back within one. But by the end of the period, it was Chalmers who was the best point guard on the floor.
The Heat dropped Game 1 in last year's finals, and the first two to Dallas in 2006 before going on to win both titles. But those early deficits came on the road, so Dwyane Wade said Sunday's game was a "must-win game" for the Heat.
They arrived to white shirts hanging on the seats that read "Larry loves Miami" with a picture of the Larry O'Brien trophy that goes to the NBA champion.
Larry's not leaving, not if the Heat keep playing like this.
They looked as good as ever in the final 15 minutes of their 100th game of the season, pouring it on and leaving Spurs coach Gregg Popovich often standing with his arms folded on the sideline, with no answers and no way of slowing down the Heat speedsters.
San Antonio had its seven-game postseason winning streak snapped, as well as a six-game NBA Finals win streak that dated to the 2005 finals.
Duncan, who started 0 of 5 in the opener before finishing with 20 points and 14 rebounds, began 1 for 5 in this one. But he never got untracked, though part of the problem was the Spurs' inability to get him the ball enough because of their turnovers.
Wade finished with 10 points and six assists. Miami committed just six turnovers.