CLEVELAND >> Kyrie Irving gritted his teeth, tightened up his left sneaker and hopped to his feet.

The pain didn’t stop him. The Celtics didn’t either.

Irving took over in the second half and finished with 42 points, LeBron James added 34 and the Cleveland Cavaliers moved within one win of an almost inevitable third date in the Finals with Golden State by rallying to beat Boston 112-99 tonight in Game 4 of the Eastern Conference finals.

With James in foul trouble, Irving stayed on the floor despite rolling his left ankle in the third quarter, when he scored 19 points in less than five minutes.

“He put us on his back and carried us,” Cavs coach Tyronn Lue said.

The defending NBA champions, who shot 71 percent in the second half to open a 3-1 lead in the series, can wrap up their third straight conference title — and a “three-match” against the Warriors — with a win in Game 5 on Thursday night in Boston.

But if Games 3 and 4 are any indication, it won’t be easy.

Fighting to keep their season alive, the Celtics aren’t giving an inch despite playing without All-Star guard Isaiah Thomas, who may need surgery on a hip injury.

The Cavs, meanwhile, wouldn’t be on the cusp of the Finals without Irving.

With Cleveland in jeopardy of dropping its second game in a row after James followed an 11-point Game 3 by picking up four first-half fouls, Irving put on a breathtaking one-man show in the third quarter.

Freezing Boston defenders with his dribble and driving to the basket whenever he wanted, Irving made six layups, two 3-pointers and a free throw in a dizzying span of 4:48. He capped his blistering 19-point outburst with a 3 in the final second of the quarter and celebrated at mid-court by pretending to put two pistols back in his holster.

“Coming out of halftime I just wanted to be aggressive, lead my guys and leave it all out there on the floor,” Irving said.

When the horn sounded to end the third, Cleveland’s players poured off the bench to greet him. They had to be careful not to knock him off his feet after Irving had put a scare through the frenzied Quicken Loans Arena crowd moments earlier when he stepped on the foot of Boston’s Terry Rozier.

Irving stayed on the floor for several seconds before sitting up and re-tying his sneaker. Nothing was keeping him out.

Celtics coach Brad Stevens was disappointed with his team’s defense on Irving, who was able to spread the floor while surrounded by shooters.

“There’s choices,” Stevens said. “I’m not sure there are good choices. When he gets going like that, he’s tough to stop. The ones we gotta look at are the ones he got at the rim.”

Kevin Love added 17 points and 17 rebounds for the Cavs, now 11-1 in the postseason.

Avery Bradley scored 19 and Jae Crowder 18 for Boston.

Lue chuckled before the game when he was asked if he planned to say anything to James following his sub-standard performance in Game 3.

“No message,” Lue said. “Just come out, be aggressive, in attack mode like he has been over the last four or five months. Nothing has changed.”

Lue should have warned James to be careful, too, as the superstar picked up four fouls and spent the final 6:46 of the first half watching from the bench, unable to help his team.

But beyond James’ foul trouble, the Cavs were having other issues as the Celtics outplayed them at both ends while opening a 16-point lead.

Irving kept Cleveland within striking distance while James was out by scoring 12 points over the final 5:11 of the second quarter.

And in the third quarter, the All-Star guard had James’ back again, keeping the Cavs on their collision course with the Warriors.