By Ryan Burton

Olympic gold medal winner Andre Ward has stopped Russian puncher Sergey Kovalev in the eighth round to win their rematch and retain his unified light-heavyweight world championship.

The 33-year-old Ward extended his perfect record to 32 wins in an impressive performance, albeit with a controversial ending, at the Mandalay Bay Hotel and Casino in Las Vegas.

Ward won the pair's first meeting last November via a unanimous but tight decision, with Kovalev aggrieved at the result having knocked the American down in round two, and the rematch was eagerly anticipated.

Again the Russian started stronger, with Ward firing off enough counter-shots to keep himself in the fight but looking weary between rounds.

The momentum began to shift the American's way as the fight reached halfway, though, with both fighters landing some fierce shots in a more open and aggressive encounter than their first bout.

And after wobbling Kovalev midway through the eighth round with a straight right hand, Ward finished the job with a series of powerful body shots, including a few borderline low blows.