Former lightweight titlist Antonio DeMarco traveled to the Philippines about a month ago to join trainer Freddie Roach in Manny Pacquiao’s training camp. Shortly after DeMarco’s arrival, he was shocked by what he heard on a phone call to his home in Tijuana, Mexico.

On that call, he learned that his 13-year-old sister, Mariana, had been diagnosed with bone cancer in her shoulder. DeMarco was distraught, but he plans to fight for his sister. He wants to make a statement to her that he is not a quitter and that she should also fight through her difficult situation, which includes chemotherapy treatments. DeMarco, whose boxing income helps support his family, also needs the purse the fight will provide.

DeMarco will challenge secondary junior welterweight titleholder Jessie Vargas on the Pacquiao-Chris Algieri undercard at the Venetian Macao’s Cotai Arena in Macau, China on Sunday morning. Given the time difference, HBO's live telecast will air Saturday night (HBO PPV, 9 p.m. ET).

"When I heard the news, I nearly pulled out of this fight,” DeMarco said through a translator. “But I decided to fight in her honor. I want her to live and be the beautiful person she is."

DeMarco, 28, said his sister’s prognosis is good, but it has still been difficult for her and their family.

“We had some tough days,” Roach said. “A couple of days he started thinking about [his sister] and he was crying. He cares about his family and his sister a lot, and it was very hard for him to be away from his family and not be able to care for her. The best way to care for her is to win this fight. I think we turned it around and turned it into a positive. DeMarco is in great shape, and I expect big things out of DeMarco.”

In October 2011, DeMarco won a vacant lightweight world title against Jorge Linares, who had dominated the fight until suffering terrible cuts and was eventually stopped in the 11th round.

DeMarco went on to make two successful defenses before he was whitewashed in an eighth-round knockout loss to Adrien Broner in November 2012. After that, DeMarco (31-3-1, 23 KOs) moved up in weight and won three fights in a row to set himself up for a shot at the Roy Jones Jr.-trained Vargas (25-0, 9 KOs) of Las Vegas.

Vargas, 25, heard about the plight of DeMarco’s sister and offered him support.

"Antonio spoke to me about his sister,” Vargas said. “This is devastating news. I am praying for her as well.”