The stunning win moved Mendoza to No. 3 in The Ring's flyweight rankings.

"Thank you very much to everyone who made this victory possible. I feel very happy and happy to have won these two titles," Mendoza (28-1, 24 KOs) told The Ring.

"In the second round I managed to knock him down twice, and he got up; showing Sandoval is a warrior. In the fifth round I knocked him down again and from there I felt that I had him, that's why I immediately went for him to achieve the goal."

The way he secured victory with relative ease belied the pre-fight talk of a mismatch. 

"I don't think it was easy, actually," he said. Mendoza was a 5/1 (+500) betting underdog who had been in camp preparing for an October 3 fight before receiving this short-notice opportunity.

"Sandoval is a great fighter and a warrior. It's just that my strategy paid off, all the work that comes from months of preparation and my corner, who gave me the right direction."

The 26-year-old Mexican was clearly and visibly stronger at 112 and not having to make 108, where he was beaten by IBF ruler Thanongsak Simsri at Tokyo's famed Korakuen Hall six months ago.

"It helped me a lot not having to push my body to the limit like in Japan. At 112 pounds, my body responds very well and without problems, I felt very strong and well recovered."

Moments after his stunning victory, a clearly emotional Mendoza broke down and cried.

"I could not hold back the tears of happiness, and also remembered the promise I made to my brother when he died," he shared. "I promised him I would be world champion, and on October 10 was able to fulfill my promise."

He celebrated the triumph at home with his wife, son, and family over a nice meal. While it is still early days, "Yoreme" has a picture of what the future may look like in mind.

"I know that this is just the beginning; now comes the most difficult part, to keep the belts, stay focused and prepare as we always do to arrive in optimal conditions for the next commitment," he said.

"I would like to first defend my belt and achieve many defenses, and why not in the future move up a category to climb more divisions and enter Mexican boxing history."