DALLAS -- How it happened: The defending champions fell apart in the fourth quarter against the Oklahoma City kids who couldn't close against the Mavs last season.

The Mavs met the first-round broom because Oklahoma City absolutely dominated after entering the final frame with a 13-point deficit.

The Thunder allowed sixth man James Harden to initiate the offense down the stretch, and the Mavs had no answer. Harden had 15 of his 29 points and three of his five assists in the fourth quarter.

Harden had a hand in every Oklahoma City point during a game-tying 15-2 run early in the quarter. He had nine points in that spurt and a pair of assists for 3-pointers by Kevin Durant, who scored nine of his 24 points in the final frame. Harden also had the dagger drive, slashing to the bucket for a layup to make it a two-possession game with 10.2 seconds remaining.

The Dallas offense sputtered to the finish line after lighting it up for 34 points in the third quarter to take its biggest lead of the series. The Mavs managed to score only 16 points in the fourth, making only four of 18 shots from the floor.

Dirk Nowitzki kept the Mavs in the game with 34 points, but he didn't get much support from his longtime scoring sidekick. Jason Terry scored only 11 points on 4-of-12 shooting in what was probably his last game in a Mavericks uniform.

In the final minute of Dallas' season, an "O-K-C" chant overpowered a "Let's Go Mavs" chant at the American Airlines Center. It was that kind of season for the 2010-11 champions.

What it means: The defending champions are done after four games. The Mavs became the fifth team to fail to win a playoff game as defending champions. The 1956-57 Philadelphia Warriors and 2006-07 Miami Heat were also swept in the first round, and the 1969-70 Boston Celtics and 1998-99 Chicago Bulls failed to qualify for the playoffs. The Thunder advance to play the Lakers-Nuggets winner in the second round.

Bold play of the game: Oklahoma City took its first lead of the second half on a Serge Ibaka slam dunk to finish a fast break. Russell Westbrook swiped the ball from Jason Terry, pushed it down the floor and fed the high-flying Ibaka for an uncontested jam. The Thunder never trailed again.

Stat of the night: Kidd outscored Westbrook, 16-12. Westbrook averaged 25.7 points in the first three games of the series, compared to 10.0 for Kidd.