Unbeaten Julio Cesar Chavez Jr retained his World Boxing Council middleweight title with a 12-round unanimous decision over Marco Antonio Rubio in San Antonio, Texas. Chavez, son of Mexican boxing legend Julio Cesar Chavez -- who looked on throughout from ringside at the Alamodome -- overcame his difficulties making the middleweight limit to claim the victory by scores of 118-110, 116-112 and 115-113. Chavez admitted that his trouble making weight took a toll. But he successfully fended off the determined challenge of Rubio, who just didn't have enough power to really hurt the champion despite his willingness to go toe-to-toe with him. Rubio, also Mexican, signalled his intentions with a straight right late in the first round and he rattled Chavez late in the fourth with a combination. But even though Chavez was tiring in the latter stages he had too much power for the smaller challenger. "I had a little problem with the weight. After the seventh and eighth rounds, my legs, I started to feel them," he said. "I was paying the price. Fortunately he doesn't hit that hard. "I don't want to battle again with the weight like I did," added Chavez, who was making the second defence of the title he won with a 12-round decision over Germany's Sebastian Zbik last June. Chavez followed that with a fifth-round stoppage of Peter Manfredo in November and is now unbeaten in 46 fights, with one drawn and 31 knockouts. Rubio fell to 53-6-1. After the bout, Chavez said that he still had no interest in moving up in weight, and that he was eager to take on the best of the middleweight ranks -- Argentina's Sergio Martinez, Mexican Antonio Margarito or Puerto Rico's Miguel Cotto. He acknowledged that he'd need to improve against the likes of the explosive Martinez. "If I fight like I did today, he would win," Chavez said. "But I know what I'm capable of and I will prepare and I will win."