Klitschko (35-2, 34 knockouts) knocked Williams down in the first, third and seventh rounds before the final knockdown.
Williams (32-4) was so desperate by the eighth round that he threw a succession of wild left hooks in a futile attempt to turn the fight around. But by this time the crowd at the Mandalay Bay hotel-casino was chanting Klitschko's name as he continued to dominate.
"I kept fighting because it was the heavyweight championship of the world, but he was just too good," Williams said.
Ringside punch stats reflected the dominance of Klitschko, who won every round on all three scorecards and landed 296 punches to 44 for Williams.
"I was surprised because he never gave up," Klitschko said.
Miguel Cotto verus
Randall Bailey
In another fight, Miguel Cotto of Puerto Rico remained unbeaten by stopping former champion Randall Bailey in the sixth round of their WBO junior welterweight title fight.
Cotto put on an impressive show, dropping Bailey with a right hand in the second round and forcing him to take a knee for another knockdown after hitting him with a left hook to the body in the third round.
Otis Grant versus
Henry Porras
Canada's Otis Grant outpointed Costa Rica's Henry Porras in a 12-round super middleweight fight in Montreal on Saturday.
"It was easier than I imagined," Grant said. "I knew he was dangerous, so I didn't take any risks."
Grant (35-2-1) received winning scores of 118-110, 118-110 and 116-113 from the three judges. The bout was the 36-year-old Montreal fighter's fourth in his comeback after a car accident that sidelined him for nearly five years.
Acelino Freitas versus
Fernando Saucedo
Brazil's Acelino Freitas outpointed Argentina's Fernando Saucedo in a 10-round lightweight fight in Sao Paulo, Brazil on Saturday.
RSS