42 of George Groves' 48 landed punches on Jamie Cox were power shots.
George Groves stopped Jamie Cox in four rounds in the first defense of his WBA super middleweight belt at Wembley Arena on Saturday.
Cox did his best to close the ring on Groves and force the close-range fight he favored, but when Groves escaped his greater power, variety and accuracy wore down his old Britain amateur teammate.
The end came in the fourth when Cox left himself open, Groves threw a big right to the body, and Cox sank to his knees, unable to rise on the count of 10 by referee Steve Gray.
Groves, who took the belt from Fedor Chudinov in May, improved to 27-3 (19 KO), and handed Cox his first loss in 25 fights.
Cox, accused by his opponent of not being a true middleweight and chosen by him at the competition's draw, began to show signs of being broken down as early as the third, even if he continued to fight with both belief and intensity.
The one-time room-mates on the Great Britain amateur squad continued to exchange, but amid the better quality of Groves' punching, the ending soon came. With Cox targeting the head and leaving himself open, the champion threw a big right hand to the 31-year-old's body that instantly sent him to his knees, unable to meet the 10-count of referee Steve Gray.
There was earlier a stoppage victory for fellow British super-middleweight John Ryder, 29. In a largely one-sided fight, he had already knocked Denmark's Patrick Nielsen to the canvas before brutally knocking him out in the fifth with three successive powerful right hands.