SAN ANTONIO -- When Julio Cesar Chavez Jr. began his professional boxing career in 2003, at age 17, he was a fighter in famous name only.
His father, stellar three-division champion Julio Cesar Chavez Sr., is a Hall of Famer and perhaps Mexico's greatest fighter. But the kid? Sure, he had the name and wore the same iconic red headband when he walked to the ring, but few thought he could fight much.
With no amateur career, he was the project of projects. But that name, so revered in Mexico, helped the kid draw crowds as he fought one no-hoper after another.
Now, all these years later -- Chavez Jr. is still only 25 -- the kid has actually developed into a good professional. He even won a world title, which he will defend for the second time against Mexican countryman Marco Antonio Rubio on Saturday night (HBO, 10 ET) at the Alamodome in a fight that many view as the toughest test of Chavez's career -- and one that has action written all over it.
In the televised opener, pound-for-pound star Nonito Donaire (27-1, 18 KOs) moves up to junior featherweight to face former titlist Wilfredo Vazquez Jr. (21-1-1, 18 KOs) for a vacant belt as HBO kicks off its 2012 boxing schedule.
"He was very, very young, not yet formed physically, and he turns pro and he goes into this boxing profession -- and you move him very, very carefully because he doesn't know very much and you have to train him," said Top Rank promoter Bob Arum, discussing Chavez's slow and steady climb up the pro ranks at Thursday's news conference.
"He was unformed. He was around boxing, of course, his whole life. But he hadn't had much experience. Now, today, he is truly an experienced veteran fighter and a world champion, and believe you me, he has demonstrated in his last few fights he deserves the accolade of world champion."
That can be debated, of course, because Chavez still hasn't faced any serious contenders.
He scored a solid win against John Duddy at the Alamodome in June 2010, in what many regard as his first serious fight. Two bouts later, he won a title by outpointing feather-fisted Sebastian Zbik, who was handed his belt when lineal champion Sergio Martinez was stripped. And in Chavez's first defense in November, he needed only five rounds to blow out Peter Manfredo Jr., who is well-known (because of his stint on "The Contender) but had little chance to win.
But now Chavez (44-0-1, 31 KOs) finally will face a legitimate contender in Rubio (53-5-1, 46 KOs), an experienced pro with good power and even more determination.
Rubio, 31, is riding a 10-fight winning streak since being stopped by then-middleweight champion Kelly Pavlik in the ninth round in 2009. Included in that streak was a huge upset last April of undefeated blue-chip prospect David Lemieux of Montreal, who had been crushing Rubio through most of the fight until he rallied for the seventh-round TKO.
"Definitely his toughest fight," said trainer Freddie Roach, who has been with Chavez since the Duddy fight. "Rubio has a lot of experience, a lot of wins, a lot of knockouts. This is definitely the most experienced and best puncher we've fought. So it is a big test for us, but I expect to pass with flying colors and go to bigger things down the road."
Chavez, who has begun to speak English, said he is anxious for the test against Rubio.
"I have a lot of experience now and I know what I have to do," he said. "Last time I fought here in San Antonio, it was a very important fight. It took me to the next level against John Duddy. Now this is the toughest fighter of my career, but I am ready for this fight. I prepared 100 percent.
"Rubio is very experienced and a hard puncher. He surprised everyone with the Lemieux win and that's why people think this is a fight. I agree it's a tough fight. But I think I've developed so much and that I am ready for these type of fighters and to beat them."
The fight with Zbik was a tremendous learning experience for Chavez, which he hopes to carry over into Saturday's fight.
"The toughest fight I've had so far was against Sebastian Zbik," Chavez said. "I think I have become a better fighter since I won the title. I work harder in the gym because I know what it takes to keep the title. My father always said titles are won and lost in training -- meaning you must be prepared. Rubio has said some awful things, but I have to deal with stuff like that. This a real tough fight for me, and I know it."
Rubio, typically mild-mannered, has trashed Chavez in the Mexican media but showed the utmost respect at Thursday's news conference.
"I've been reading what he's been saying, but he didn't say anything to me," Chavez said. "I met him twice and he never came up to me and said anything. Words, they go with the wind. I don't care about that. We'll show him Saturday night what we think about him."
Rubio has predicted a sixth-round knockout, which seemed to annoy Chavez.
"He keeps saying he will knock me out in six rounds," Chavez said, "and I don't think he is giving me the respect that I deserve."
Rubio has also repeatedly dissed the legitimacy of Chavez's title, as many have, given that it was stripped from Martinez and the fact that, even when a fight between the two was ordered, Top Rank found a way to avoid it.
"We both punch hard and there is going to be a knockout. I think we both realize that," Rubio said through a translator. "This is the best opportunity for me to win a WBC middleweight championship, which is why I have been calling for this fight. I told some press in Mexico that I don't believe he is a real champion, but he has the belt and I know he will fight hard to keep it.
"I'm very happy and very motivated to be here in San Antonio for what I consider to be the most important fight of my career. It's a great opportunity, and I believe that this is a very winnable fight for me. I have trained very hard in Mexico and, with all due respect to Julio Cesar Chavez Jr., who is a very good boxer, he is not even the best champion in the division and has not fought the level of competition that I have. After I lost to Kelly Pavlik, I made some changes in my team and it really has paid off with a long winning streak that has helped me to get this new opportunity to fight for a world championship. No one has given me anything in my career. I have earned everything, and this fight and win Saturday will be the crowning achievement of my professional career."
Chavez and Roach understand why many question the validity of the title Chavez owns, but they also know a victory over Rubio will go a long way toward easing the criticism.
"I think it makes a great statement for [Chavez] and legitimizes the title a little bit more," Roach said. "But the thing is, I'm not looking for the knockout. I told him go out there and win every round, one at a time. If the knockout comes, it comes. If you go out there looking for a knockout, you're liable to get hit in the exchange. I want him to fight a smart fight. That's the most important thing to me."
Said Chavez: "It's important to me to show I am the top 160[-pounder] in the world, and I know that I have to fight these type of guys and beat them. I know this is going to give more credibility to my career and to my championship. I also want Martinez. He's another guy who's been talking a lot. Nothing in front of me, but I hear him talking. I'm ready. If they say that's the next fight, that's the next fight. I'm ready for whenever Arum makes it.
"But with Rubio, a lot of people think he will beat me, so that gives a lot of credibility to me when I beat him."
Dan Rafael is a boxing writer for ESPN.com. Follow him on Twitter @danrafaelespn.