The play occurred with 2:08 remaining in the third quarter when James and Cavs backup forward James Jones double-teamed Harden, trying to force a jump-ball situation. Harden fell to the floor and, while he was still lying on the court, kicked at James.
"Obviously that's not a basketball play," James said afterward. "Obviously, the league will probably take a look at it. I have no idea why he would do that, but two competitors just trying to go at it, and he won this one."
Asked whether there was any history of bad blood between he and Harden, his 2012 U.S. men's basketball Olympic teammate, James told ESPN.com, "No. That came out of nowhere."
Said Harden: "He's a very good friend off the court. Obviously it's a battle and just trying to win a basketball game. Just competition against one of the best players in the world."
Asked whether he expected to be disciplined by the league, Harden said, "No."
Later Sunday night, the Rockets appeared to take another jab at James, posting on their Twitter account:
There is precedence in a situation like the one involving Harden and James. Miami's Dwyane Wade was suspended one game during the 2012-13 season for kicking Ramon Sessions in the groin with his shin, even though he was allowed to finish that game against Charlotte after the kick.
Cavs coach David Blatt chose not to elaborate on the call, saying, "I didn't get a great look at it. I don't want to give you an uneducated opinion."
Jones, however, was pointed in his view of Harden's involvement.
"It was an intentional groin kick," Jones said. "It was an intentional kick to a troublesome area."
In the ensuing commotion after the kick, Cleveland's Tristan Thompson and Houston's Corey Brewer were whistled for double technical fouls.
James was called for a technical foul of his own earlier in the third quarter, along with Patrick Beverley, for pushing Houston's point guard after the two got tangled up and fell to the floor.
Rockets coach Kevin McHale brought up that sequence with Beverley when asked about the Harden flagrant.
"[Earlier in the game], I looked up and they gave Patrick a technical [along with] LeBron James for [James] pushing him down," McHale said. "So I didn't even look at [questionable calls] after that. I thought if you get a technical for a guy pushing your head on the floor, why the hell am I going to watch the video [of Harden]? Anything can happen at this point. I didn't even see [the kick]. Someone said that, I didn't see that. I was just trying to concentrate on trying to coach."
Beverley, a former training camp invitee with Miami when James was on the team, said he planned to get dinner with the four-time MVP on Sunday night despite the skirmish they were involved in.
"I thought I took the charge, but the refs called block," Beverley said. "I took the charge and I tried to get up, and he pushed me down. Tussling. Just basketball."
ESPN.com's Calvin Watkins contributed to this report.
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While fighting for the basketball, James Harden kicked LeBron James in the groin.
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Houston Rockets guard James Harden talks to Lisa Salters after the Rockets defeated the Cavaliers in overtime.