LAS VEGAS -- Manny Pacquiao caught Shane Mosley early, then chased him the rest of the night. Not much more he could do against an aging fighter who seemed only to want to survive.

Pacquiao won a lopsided 12-round decision Saturday night, retaining his version of the welterweight title in a fight that was roundly booed over the late rounds because Mosley refused to trade punches.

Pacquiao won every round on two ringside scorecards in extending the remarkable run that has made him the most exciting fighter in the sport.

Frankie Edgar has a lot more in common Manny Pacquiao than he does with Arturo Gatti. Chris Trotman/Getty Images

He also won the ungrudging respect of a veteran fighter who has been in with some of the best in the world.

"I fought the best fighter in the world," Mosley said. "He has exceptional power, power that I've never been hit like this before."

Pacquiao knocked down Mosley with a left hook in the third round, a punch that sapped Mosley's willingness to engage. Pacquiao ran after Mosley the rest of the fight, but the former champion who has never been stopped in 18 years in the ring managed to stay away enough to finish the 12th round upright.

Pacquiao won 120-108 on one scorecard, 120-107 on a second and 119-108 on the third. The Associated Press had him winning 118-110.

For Mosley, the fight was strikingly similar to his bout a year ago against Floyd Mayweather Jr. -- except this time Mosley didn't even land a big punch like he did early on against Mayweather. Mosley's biggest moment on this night came when referee Kenny Bayless mistakenly ruled that Mosley knocked Pacquiao down in the 10th round when he didn't even hit him with a punch.

The knockdown that wasn't spurred Pacquiao on as he went after Mosley the rest of the round and again in the 11th. By then the crowd was cheering "Knock him out! Knock him out!" but Pacquiao -- bothered by a cramp in his left leg since the fourth round -- didn't have enough to finish him off.

"It wasn't my best performance," Pacquiao said. "I did my best. I did not expect this result."

The closest Mosley got to Pacquiao all night came at the beginning of the 12th round when he hugged Pacquiao before the two went about finishing their business. That took some of the fire out of Pacquiao, who didn't really go after the knockout in the final round.