Bernard Hopkins defended his undisputed world middleweight championship for the 19th time on Saturday, knocking out Oscar De La Hoya in the ninth round of their scheduled 12-round bout in Las Vegas, Nevada.

The tentative contest came to a sudden end 1:38 into the round when Hopkins hit De La Hoya with two body shots, knocking the challenger to the canvas in obvious pain.

When De La Hoya rolled over onto his back, referee Kenny Bayless stopped the fight, which had been fairly uneventful until the knockout.

Both men appeared conservative in the early going, with neither willing to take any risks until Hopkins connected with his two shots to the liver area to end the bout.

De La Hoya never came close to beating the count, giving Hopkins the biggest win of his career and his biggest payday.

However, the challenger earned the lion's share of a purse believed to be worth around $40 million, the richest non-heavyweight fight in boxing history.

While Hopkins celebrated in the ring, De La Hoya pounded on the canvas in frustration after being knocked out for the first time in his career.

He was fighting as a middleweight for just the second time.

"I felt the urgency," Hopkins said. "I knew Oscar was boxing a good boxing game, and I knew I had to turn it on. I give him credit, he didn't run and he came to box. It surprised me."

Two judges had Hopkins ahead (79-73 and 78-74) on their scorecards at the time of the knockout while the other judge had De La Hoya in front (77-75).

The 39-year-old Hopkins remains undefeated in the past 11 years and justified his tag as hot favourite.

He improved his record to 45-2-1 with 32 knockouts while the 31-year-old De La Hoya fell to 37-4 with 29 knockouts.