LAS VEGAS, Nevada - Philippine boxing champion Manny Pacquiao, who hasn't lost a fight in six years, said the only thing preventing him from knocking out Shane Mosley was a cramp in his legs.

Pacquiao successfully defended his World Boxing Organization welterweight title for a second time, winning by a unanimous decision after 12 rounds in front of a sold-out crowd at the MGM Grand Hotel and Casino on Saturday.

Pacman floored Mosley in round three - the challenger's first knockdown for nine years - but quickly felt a twinge in his left leg.

"After the first knockdown I felt I had him and I got him and that I could finish him," Pacquiao said. "But the next round I have a problem with my left foot it is tight.

"I told my coach to press my legs because they are too tight."

Pacquiao's trainer Freddie Roach said his fighter had problems with cramping before but usually in training sessions and not in the middle of bouts.

They have tried different remedies including stretching and other exercises but nothing worked against Mosley.

Despite the pain 32-year-old Pacquiao put on an impressive performance Saturday, dominating from start to finish.

The only blot on an otherwise convincing display was a knockdown awarded to Mosely in the 10th round, a decision the referee later apologised to Pacquiao for.

Mosley claimed he would expose Pacquiao's weaknesses but there was no turning back the clock for the 39-year-old former world champion, who looked every bit his age Saturday.

Mosley's main goal appeared simply to survive the onslaught launched by the Philippine icon and to keep his record of never being knocked out intact.

The challenger maintained that record but was booed loudly for his negative tactics, especially in the eighth and ninth rounds.

Even Pacquiao gave him the cold shoulder in the ring after the final bell and then questioned Mosley's game plan at the post-fight news conference.

"I was expecting him to fight with me for at least five of the 12 rounds so we can test each other's power," Pacquiao said. "I did my best to try and give a good fight. What can I do? It is not my fault."

Agence France-Presse