Garcia (34-1, 20 KOs) did his best work in the center of the ring, where he was able to box at a distance and fend off Rios' furious pace. Rios, who returned in 2017 from a brief retirement, was most effective against the ropes where he gave Garcia trouble with his constant pressure and his favorite weapon - a wide overhand right.
In the ninth round with the fight where Garcia wanted it - in the center of the ring - the Philadelphia native landed a huge counter right to the chin that sent Rios flat on his back. Rios beat the count, but was clearly wobbly on his feet, forcing referee Kenny Bayless to halt the contest at 2:25 of the ninth.
"I felt the ring rust a little bit in the beginning," said Garcia, who recorded the 20th knockout of his career. "He's a good inside fighter and he was giving me some good uppercuts. I felt good, it was a good nine rounds. He came to fight. I came to box, I did that. I came to bang, and I gave the fans what they wanted - a knockout.
"I just noticed when I was getting my punches off he was standing right in front of me and I just let it go. As soon as I got the fight in the middle of the ring where I wanted I landed good shots. I was just letting my hands go and the punch landed."
After the fight, SHOWTIME Sports reporter Jim Gray asked Garcia how the loss to Thurman affected him.
"The loss was tough. I have the mindset of a winner," Garcia said. "I hate losing. I took it like a true champion and I bounced back like a true champion.
"I would love the rematch with Keith Thurman. It's on him. Whenever he's ready we'll fight."
WBA and WBC Champion Thurman isn't the only potential blockbuster on the horizon for Garcia. Former welterweight champion and the WBC's No. 1 ranked contender Shawn Porter looms for what would be a massive showdown between the former champions.
After the fight, Rios (34-4-1, 24 KOs) was frustrated with the stoppage and promised to return.
"I'm mad. I don't like going out like that," Rios said. "I'm a warrior, I got back up and I wanted to continue. I guess they stopped it but I'm mad. I'm mad because I got up.
"I was doing good. I got lazy with the jab and he came over with the right hand and he caught me. I'm a warrior. I'm still in the game. I didn't think he did much to hurt me. I think it was closer than what he thought it was. He just caught me with a lucky punch. I was ready to continue."