The unknown underdog upset Saudi prospect Ziyad Almaayouf by unanimous decision in their eight-round welterweight fight on “The Ring 6” undercard at Madison Square Garden. Castillo overcame a first-round knockdown and landed the harder punches to Almaayouf’s head and body over the final six rounds.

Judges Mark Consentino (77-74), Glenn Feldman (78-73) and Steve Weisfeld (77-74) all scored for Ecuador’s Castillo. FanDuel listed Riyadh’s Almaayouf (7-1-1, 1 KO) as a 13-1 favorite over Castillo (6-2-1), who had been 0-2-1 in his previous three fights.

Castillo doesn’t have any knockouts, but the 29-year-old Miami resident was clearly the stronger puncher and walked through virtually everything Almaayouf, 25, threw at him after the second round. He caused bruising beneath Almaayouf’s left eye and wore him down during the second half.

Castillo unloaded punches on Almaayouf throughout a one-sided eighth round. He failed to make adjustments to neutralize Castillo’s pressure and isn’t a big enough to puncher to produce the knockout he clearly needed in the final three minutes.

Castillo’s left to the body and several right hands to the head made Almaayouf retreat with under 40 seconds to go in the seventh. Almaayouf seemed to fade by then and couldn’t keep Castillo from coming forward to unload on him.

A left-right combination by Castillo dislodged Almaayouf’s mouthpiece again a little less than a minute into the sixth. Castillo was the busier, harder puncher as he continued to build on his momentum.

Almaayouf started throwing punches again in the fifth, which was more competitive than the fourth.

Castillo continued cracking Almaayouf with power shots in the fourth. With blood flowing from a cut around his left eye, Castillo caught Almaayouf with several effective punches in the third, including a right that made him reset his feet.

Almaayouf’s right to the side of Castillo’s head sent him to one knee with 18 seconds to go in the first, but he recovered quickly.