LeBron James admitted he is feeling the strain of a gruelling NBA playoff campaign after the Cleveland Cavaliers' game-five defeat to the Boston Celtics.

James looked fatigued as the Cavs slipped 3-2 behind in the Eastern Conference finals series following Wednesday's 96-83 loss to the Celtics in Boston.

The Cavs superstar flirted with a triple-double after posting 26 points, 10 rebounds and five assists, though the 33-year-old looked off the pace in the second half.

Speaking afterwards, James – who inspired wins over the Indiana Pacers and Toronto Raptors in the postseason – told reporters: "I had my moments.

"But I think everybody at this point is tired, worn down or whatever the case may be."

Asked if he was under the weather, James responded: "No. I'm fine.

"I didn't mention the fatigue, either. You guys did."

Tyronn Lue was quizzed about James following the loss, the Cavs head coach adding: "He looked a little tired to me."

Pressed as to whether he was concerned, Lue said: "No concerns. You've got to be ready to play now. No concerns."

The Cavaliers are now on the cusp of elimination, which could spell the end of James' career in Cleveland amid uncertainty over his future.

"Our focus, LeBron's focus, is to win. That's all we're focused on right now," Lue said.

"Anything else doesn't matter right now. We've got to be ready to play game six. That's all we're worried about."

The Cavaliers are back in Cleveland for game six at Quicken Loans Arena on Friday.