MIAMI | Wed Dec 19, 2012 10:20am EST

(Reuters) - LeBron James says he is engaged in a "vendetta" against himself as he pushes himself to reach his physical peak and win a second straight NBA championship with the Miami Heat.

On Tuesday, James, last season's Most Valuable Player, who spent his off-season playing in the London Olympics, was on court against the Minnesota Timberwolves for 42 minutes.

"He wasn't even breathing heavy," said Heat head coach Erik Spoelstra when asked about the longer than expected regular season minutes for his team's star.

"He's been biking so much on his own. He biked to the game the other day. He's turning the corner right now in terms of his conditioning. It's world class."

When Miami hosted the Washington Wizards on Saturday, James chose to cycle to the team's daytime shoot-around and the 45 minutes back home before doing the same in the evening for the game.

That's three hours of road cycling on a day in which he scored 23 points in a 102-72 win for Miami.

The focus and the fitness are evident on court. James will no doubt make a step up in intensity for the playoffs but he's hardly going through the motions in the regular season having made at least 20 points in each game.

"I felt great," he said after Tuesday's game, "I felt like I didn't get tired tonight. One time in the second half I felt like I could have played again if we had too. I have been biking a little more than usual.

"It is fun but it is also condition and cardio. It is something I like to do," he added.

James says he is too far "in the zone" to notice any public reaction to one of the world's leading sportsmen cycling around the streets of Miami a few hours before a game.

Perhaps even more revealing of his psyche was James's behavior after the December 6 home loss to the New York Knicks.

James left the court and went to the gym, not appearing for his post-game media duties until he had finished an evidently strenuous work-out.

It sounds an extreme approach, even for a professional athlete, but James appears to need to challenge himself, even, or perhaps especially, when he is still so dominant on court.

"I don't know. I just have a drive to continue to improve and just continue to fight through it. Yesterday (Monday) I was extremely tired but I got some more work in.

"As a leader I just want to continue to push through it. I have a personal vendetta against myself right now," he said with a grin.

Vendetta was an interesting choice of words for a man who became a hate figure for many NBAfans following his free agency move to Miami but who has silenced those voices with his outstanding displays this year.

But at 27, James is clearly concerned with not standing still in terms of his performance and his physical fitness.

"I want to get better. I want to maximize everything I can and not waste an opportunity each and every day to compete and get better as a player.

"I want to be the best, you have to push the button sometimes".

(Reporting By Simon Evans, editing by Ed Osmond)