Parker, who repeatedly declared "I'll be ready to go" to questions about his health before Game 4, said afterward he "definitely got fatigued" in the second half.
That's when Parker took only four shots and made none. It was an abrupt reversal after a brilliant first half, when Parker looked like the NBA's best point guard just as he did at the start of the series.
By the end, he settled for 15 points and nine assists in Game 4.
"It was kind of weak. I didn't know what to expect," Parker said. "So the first three, four minutes I was kind of testing it, and the first half it felt OK. And the second half I think I got fatigued a little bit."
Then he threw in a silver lining.
"Overall I'm just happy I didn't make it worse," he said.
The series is tied 2-all heading into the final home game for San Antonio on Sunday night. Parker said he'll be OK to play after packing the next two off days with treatment, but the Spurs might need him to be spectacular again to win a fifth championship.
Spurs coach Gregg Popovich said health wasn't Parker's problem.
"Miami did a great job on him," Popovich said. "They doubled. They got it out of his hands and other people had to play."
Parker shot 7 of 16 and reached the foul line just three times. It was a sudden disappearing act after scoring or assisting on 12 of San Antonio's first 16 field goals.
Despite the lingering problems from his hamstring, Parker didn't make it obvious. He played 31 minutes and well into the fourth quarter before the Spurs emptied their bench as the Heat pulled away.
Parker was hurt in the second half of Game 3, and an MRI taken Wednesday revealed a Grade 1 strain. That's the mildest level, but Parker wouldn't commit to playing Thursday night until the morning shootaround.
The way he started the game, it appeared all the concern about his hamstring was silly. He shot 58 percent in the first half, and had already outscored his listless performances in Games 2 and 3.
Miami stars LeBron James and Dwyane Wade had rebounded, too, making this series a thrilling matchup of dueling superstars again.