Floyd Mayweather beat Victor Ortiz in controversial fashion in Las Vegas to maintain his perfect record and win the WBC welterweight title.

The American (now 42-0) knocked Ortiz (29-3-2) out with a big left followed by a huge right in the fourth round while Ortiz had his guard down after being penalised a point for an attempted headbutt.

Mayweather's decision to mercilessly punish Ortiz's lack of awareness was questionable from a sportsmanship standpoint but well within the rules as Ortiz failed to abide by the old boxing mantra of "protect yourself at all times."

Ortiz, who had struggled to cope with Mayweather's speed and accuracy up until that point, could easily have been disqualified for his blatant attempt to headbutt in the exchange before the fight-ending punches.

At the restart he went to Mayweather for an apologetic embrace which Mayweather seemed to be accepting.

Both fighters touched gloves but with the referee Joe Cortez then looking at the timekeeper and Ortiz seemingly looking at the referee, Mayweather decided to unleash a left hook on Ortiz that stunned his opponent before following it with a brutal right that sent Ortiz to the canvas.

Ortiz was counted out after that onslaught and Mayweather, who had the crowd against him all night, was booed by the majority of the audience when replays of the "sucker-punches" were shown.

The drama did not stop there either as Mayweather was involved in a heated exchange with HBO presenter Larry Merchant during his post-fight interview in the ring.

Mayweather, unhappy with the line of questioning from the veteran presenter, ranted to Merchant "you don't know sh*t about boxing" to which Merchant replied: "If I was 50 years younger I'd kick your ass."

The manner of the victory will take away from what up until that point had been a classy performance from Mayweather who showed no signs of ring rust after a 16-month lay-off.

The American used his trademark counter-punching skills to pick off Ortiz during the first three rounds with some pin-point right hands.

Ortiz was having a bit more joy at getting through to Mayweather in the fourth but was also still being caught far too easily himself before the dramatic finish to the fight near the end of the fourth round.

On the undercard, Mayweather's protégé Jesus Vargas (17-0) only just held onto his unbeaten record with a contentious split decision victory over Josesito Lopez (29-3) in a Light Welterweight clash.

Vargas, who Mayweather says Amir Khan must beat before fighting him, was given the fight 95-94 96-93 by two judges with the third judge scoring it 96-93 Lopez, but many at ringside thought Lopez should have been granted the decision.

Mexican legend Erik Morales (52-7) became a four-weight champion after beating Pablo Cesar Cano (21-1-1) with a TKO at the end of the 10th round to win the WBC lightweight title.

Cano did not come out for the 11th round after a severe cut under his left hand eye left him unable to continue.

Meanwhile, in Los Angeles but still part of the same "Star Power" promotion, an explosive barrage of punches from Saul Alvarez (38-0-1) in the sixth round saw him defend his WBC Light Middleweight world title in a tough battle against Alfonso Gomez (23-5-2) with the referee stopping the contest.

Seán Fay / Eurosport