An exhausted LeBron James got back to work Friday and was upbeat after a battle with cramps wrecked Game 1 of the NBA Finals for him.

James was jovial and completely unbothered by the criticism that landed on him in the 12 hours since the end of Thursday’s game, and said he expects to be fine when the Heat return to AT&T Center for Game 2 on Sunday (8 p.m., ABC). Miami’s medical staff will continue treating and monitoring him, but he should not need additional fluids intravenously after receiving nearly three bags immediately after the game.

“If I had to say today I would probably be out on Sunday; I probably won’t play,” he said, pausing to see if anyone fell for that line. “No, I’ll be all right. I should be 100 percent on Sunday.”

James has had trouble with cramps since high school, but these were the result of unusual circumstances. The air conditioning unit at AT&T Center malfunctioned Thursday, and the temperature in the arena was believed to be around 90 degrees. The Spurs apologized for the outage in a statement and said the system is now “fully operational” after repairs.

James lasted just 33 minutes Thursday, the least he has played in this postseason other than a game in which he had foul trouble, and one blowout of Indiana in which he missed most of the fourth quarter. Miami led 86-84 when his pain intensified and fell behind by four while he was out.

He came back for 34 seconds and got a layup to cut the Spurs’ lead to 94-92 before limping to the bench with 3:59 remaining. San Antonio caught fire while James was out and blasted the Heat 110-95 for a 1-0 series lead. James finished with a respectable 25 points, six rebounds and three assists.

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Miami has been aware of his cramping issues the entire time he has been with the team and has procedures to prevent the problem. It has not surfaced often, though it notably cost him time in last month’s Eastern Conference finals and in Game 4 of the 2012 NBA Finals.

James’ pre-game routine and the Heat’s handling of his dehydration were the same as ever, but coach Erik Spoelstra said the episode could not be compared to relatively minor flare-ups in the past.

“Last night was so extreme,” he said. “That’s the toughest part for people to understand. He was burning through his fluids and calories at an extraordinary rate. It needed to go to another extreme level. He took seven cramping pills. He was taking electrolytes the entire game, building up his fluids. Every timeout we took him out of the game, far more frequently than we ever have in the past.”

None of those solutions worked, and James was irate about missing the key stretch of such an important game.

“I was angry, I was disappointed in myself,” he said. “I mean, I did everything that I needed to do to prepare for this game, prepare for this moment and to feel like my body failed me — I was angry in the fact that I couldn’t help my team get over the hump in a huge Game 1, wanting to make a statement.”

He was far less furious when informed that he was being crushed for that as though it were an actual failure.

He was briefed on the reaction by media, former players and even the official Twitter account of Gatorade (James has an endorsement deal with Powerade), but saw no need to defend himself.

“What everybody has to say — you guys should know me by now,” he said. “I don’t care. I really don’t care what people say about me. I don’t care about that sports drink group — I’m not even going to say their name. This is about the Spurs and the Heat, and it’s not about everybody else, man. I don’t care.”

His team took up the fight for him.

Spoelstra reiterated that James tried to go back into the game and was told no, stressing that “99.9 percent of people have never pushed themselves to that level.” Shane Battier said those who likened James’ cramps to their own need to realize that an NBA game is more taxing than Zumba classes. Chris Bosh thought “the cramp game” will become little more than a funny story when they look back on it decades from now.

And more importantly, they anticipate a ferocious comeback in Game 2.

“He’s our guy, he’s our leader,” Rashard Lewis said. “We’ve got his back 100 percent and we’re pretty sure he’ll be ready to play 48 minutes if he has to on Sunday.”