Serrano added the WBA featherweight title to her WBC, IBF, WBO, IBO and The Ring haul ahead of her bout with undisputed lightweight champion Taylor in Dublin on May 20th.
The Puerto Rican was made to go the distance by Mexican Cruz, though, before eventually claiming victory after the three judges scored 98-92, 98-92, and 97-93 in her favour.
"Erika was a Mexican champion, she worked hard, that's what we trained for," Serrano said afterwards.
"I didn't expect anything less than what she gave me this night.
"They sent me back to basics and I was listening to my corner who asked me to go 1-2, which worked."
Serrano came into the contest as the favourite but was stumped by an aggressive Cruz, who was riding a winning run of 14 fights.
The pair exchanged punches at a consistent rate and Cruz suffered a nasty cut in the third round after an accidental clash of heads.
However, she powered on but was unable to slow the progressing Serrano, who almost floored her in the sixth.
Despite taking the bout the distance, there was little more Cruz could do to prevent a first defeat in almost seven years.
Elsewhere on the card at Madison Square Garden, America's Alycia Baumgardner became undisputed champion in the super-featherweight division with victory over France's Elhem Mekhaled.
