Immediately after the victory, Marquez called for a third fight with eight-weight world champion Manny Pacquiao of the Philippines. Pacquiao and Marquez drew their first bout in 2004, and Pacquiao won a split decision in a rematch four years later.
“We know Pacquiao is avoiding us,” said Marquez afterwards.
“A third fight is what the public wants.”
After a fast-paced opening two rounds, in which Mexican Marquez landed blistering counter combinations as his Australian challenger pressed forward, Katsidis, 27-3 (22 KOs) dropped Marquez in round three with a short left hook.
The champion appeared badly hurt but was able to survive the onslaught and by the end of the round was troubling his opponent with pinpoint punches to body and head.
Katsidis continued to swarm forward aggressively, throwing a high volume of punches, but Marquez, 52-5-1 (38 KOs) responded with devastatingly accurate counters.
In the ninth, Marquez stepped up the pressure, landing with combinations and backing Katsidis across the ring. As Marquez continued to land at will, referee Kenny Bayless called a halt to the contest.
Katsidis was fighting despite the death of his brother, Stathi, last month. Stathi, a professional jockey, was found dead in his Brisbane home on October 19 from as yet undisclosed causes.
“I went through a lot to make it to this fight,” Katsidis admitted afterward.
“It was a hard thing to even make it through this fight,” he added tearfully.
Earlier, in Oakland, undefeated American Andre Ward retained his WBA super middleweight title with a unanimous decision victory over Cameroon-born Sakio Bika.
Ward, 23-0 (13 KOs) won every round on one scorecard and ten of twelve on the other two, but despite the wide margins, the victory was hard fought.
At the end of the contest, both fighters were bleeding from cuts by the eyes, and Bika, 28-5-2 (19 KOs) also from the nose.
Ward next meets Arthur Abraham in the semi-finals of the “Super Six” World Boxing Classic super middleweight tournament.
Earlier on Saturday, Germany-based Armenian Abraham lost a wide decision in Helsinki to Britain’s Carl Froch, who regained the WBC title and will face Jamaican Glen Johnson in the other semi-final.
