By Sportsmail Reporter

Last updated at 4:00 PM on 18th September 2011

American Floyd Mayweather claimed the WBC welterweight title with a controversial victory over Victor Ortiz in Las Vegas - before getting involved in an ugly spat with HBO's Larry Merchant.

Mayweather knocked out Ortiz at the end of the fourth round to extend his unbeaten record to 42 fights.

But the 34-year-old looked to land the knockout blows while Ortiz was not ready after being penalised for a headbutt seconds earlier.

No defence: Victor Ortiz did not have his hands up when Floyd Mayweather launched his attack

After a quiet opening, the fight burst into life in the fourth round with Ortiz getting Mayweather on the ropes without quite being able to find a decisive blow.

Just before the bell Ortiz had Mayweather on the ropes for a third time, only to be penalised for a headbutt.

The 24-year-old then seemed to be apologising and was not protecting himself as Mayweather landed two punches - the second a big right hand - to end the fight.

'I got hit by a dirty shot. The rule is protect yourself at all time,' Mayweather insisted in a post-fight interview with Merchant.

Fighting talk: HBO's Larry Merchant told Mayweather: I wish I was 50 years younger and I would kick your a**

'We are not here to cry and complain about what he did or I did dirty.'

When Merchant tried to press Mayweather about the controversial ending, the boxer raged: 'You never give me a fair shake. You don't know s*** about boxing.'

Merchant replied: 'I wish I was 50 years younger and I'd kick your a**.'

Speaking at a post-fight press conference, Mayweather later added: 'I was always told to protect yourself at all times.

No more: Ortiz is counted out in Las Vegas after Mayweather's knockout one-two

Attack: Ortiz had had Mayweather in trouble with a series of punches in the run-up to the headbutt

'My mouth was split open. I said keep it clean. Once you touch gloves, it is go time.

'We came together to fight. It is fight time. You want to do me dirty and then two minutes later you want to be my friend? This is the hurting business.

'Without the fourth round, he was going to get knocked out anyway. I was fighting stronger. I was keeping my composure and sticking to the game plan.

Heads you lose: Ortiz attempted to headbutt Mayweather while he had him on the ropes

'Floyd Mayweather isn't ducking and dodging anybody. If he wants it, he can have a rematch.'

Ortiz, for his part, said of his headbutt: 'I fouled Floyd, I apologised in the ring and apologised after the fight as well. I would really like the rematch.

'I thought the ref (Joe Cortez) called a break and I am pretty sure he did and then I was like "whoa, whoa", and then I woke up after.'

Mayweather also shrugged off renewed calls for a superfight with Manny Pacquiao, insisting his Filipino rival 'has to get his business in order' before the two can meet.

Face off: Ortiz feels the full force of Mayweather's jab before the knockout blow

The failure of boxing's two leading fighters to get it on is the longest-running saga in the sport but remains as far off as ever with Mayweather still insisting on extra stringent drug tests and Pacquiao seemingly intent on pursuing other plans.

Mayweather victory over Ortiz did not stop talk quickly turning to the long-awaited pound-for-pound showdown.

Plenty to say: Mayweather explains himself after the fight at the press conference

In Floyd's corner: Mayweather's finacee, Shantel Jackson, watches on at the post-fight press conference

Champion: Mayweather won the WBC welterweight belt with his fourth-round stoppage in Las Vegas

'Pacquiao is famous because he is attached to my name,' Mayweather ranted. 'When they say Pacquiao, they say "that's the guy they are trying to get to fight Floyd Mayweather". When they say Floyd Mayweather, they say the greatest ever.

'How can Pacquiao offer me anything? I do the offering.

'He has to get his business in order. When he fights, he gives up 30% of his check, I get 100 per cent.'

Self-styled 'Money' Mayweather added: 'I just want an even playing field. That's it. If you have nothing to hide, then take the test.

'With or without Pacquiao, any time Floyd Mayweather goes out there he is going to make 70 million dollars. I don't need him.'

Mayweather is now unbeaten in his 42-fight career after this weekend's win in Las Vegas.

Also on the Golden Boy Promotions show, Mexican legend Erik Morales became a four-weight world champion by taking the vacant WBC light-welterweight title against over-matched Pablo Cesar Cano with a 10th-round retirement.

The win keeps Morales in contention for a clash with IBF/WBA champion Amir Khan.