A near-sellout crowd of 15,000-plus fans at the Toyota Center in Houston saw Marquez use his experience and ability to overcome Diaz's youthful and effective aggression.
"I had to work and box and use head and body movement to avoid him,'' Marquez said. "After the fourth or fifth round, I saw that my body punches were making him tender to the body. Fortunately, the knockout came."
Diaz, 25, was flat on his back when the fight was halted at 2:40 of the ninth round. Marquez delivered a right hand to knock him down for the second and final time just moments after he landed a wicked right-left combination to floor Diaz the first time.
With blood streaming from a cut over his right eye, Diaz was unable to fend off Marquez's show-stopping assault.
"I got caught with a good punch," Diaz (34-2-0, 17 KOs) said. "After I got cut, blood was in my eye and I couldn't do anything about it. It was bothering me. I couldn't really see the punches."
Marquez (50-4-1, 37 KOs) entered the fight universally considered the No. 2 fighter in the world behind Manny Pacquiao, an opponent he twice has gone the 12-round distance against.
Diaz, the former unified lightweight champion, has fought some of the best talent the division has to offer but was clearly stepping up a level in facing the 35-year-old champion from Mexico City.
In another title fight on the HBO World Championship Boxing card, Indonesia's Chris John staved off a furious finish by Houston's Rocky Juarez to retain the WBA featherweight title with a 12-round draw. All three judges scored the fight 114-114 after each gave Juarez the last two rounds.
Juarez (28-4-1, 2 KOs) was hoping to finally ascend to a world title after losing in three previous tries. But John used a nearly 2-1 advantage in punches to control most of the fight while Juarez, who sustained a cut over his left eye=2 0in the fifth round, perhaps waited too long before he got down to serious business. "It was a great show and I fought with all my heart,'' Juarez said.
"I thought I won the fight. It was unfortunate I wasn't able to win a world title here in my hometown, but at least it didn't put a loss on my record. I may have given up some of the middle rounds and that was my fault. But Chris John is a great fighter."