ATLANTIC CITY, N.J. — Sergio Martinez and Paul Williams landed punch after punch a year ago in an instant classic, one of those rare occasions when two of the best boxers in the world come together to create something special.

ATLANTIC CITY, N.J. — Sergio Martinez and Paul Williams landed punch after punch a year ago in an instant classic, one of those rare occasions when two of the best boxers in the world come together to create something special.

Their rematch Saturday night didn't last nearly as long, and only one punch mattered.

Martinez landed a devastating left hook early in the second round, knocking out one of boxing's most feared punchers while retaining his middleweight championship.

The sudden outcome left a lively crowd at Boardwalk Hall in stunned silence.

"I started to attack, and when I did, we knew he was going to make a mistake, because he always makes mistakes," Martinez said. "He left me a lot of room to come in and hit him."

Williams was looking to land his own hook and instead walked right into the punch, which landed flush on the chin and sent the challenger face-down onto the canvas. Martinez immediately ran across the ring to celebrate while doctors rushed in to tend to Williams.

It took several minutes before he finally got to his feet, and by that point, ring announcer Michael Buffer was already announcing the end at 1:10 of the second round.

"I got caught with a punch," Williams said while being tended to by paramedics.

In their first encounter last December, Martinez and Williams traded knockdowns in the first round before trading blows for 11 more. Williams ultimately won that fight by close and somewhat controversial decision, and Martinez had been saying all along that he planned to make sure this one didn't end up in the judges' hands.

He sure accomplished that.

"I didn't want the judges to rob me this time," said the 35-year-old Martinez, one of the rare fighters entering his prime this late in his career.

"He said, 'I'm going to knock him out. The fight is not going to go seven rounds, I'm going to knock him out,'" Martinez promoter Lou DiBella said, unable to contain a smile. "I was worried like a maniac and he said, 'No, I'm knocking him out. Relax.'"

The only real drama came before the fighters made their way to the ring, when DiBella was furious that his guy as champion was forced to work out of the blue corner. That side of the ring had not delivered a winner all night, at least until the main event.

"I mean, in my heart, I thought he was going to win, but that was one of the great knockout punches of another great fighter I've ever seen," DiBella said. "Williams is a brilliant fighter, but Martinez's speed and angles, that punch — that punch would have knocked anyone out."