NEW YORK – Amanda Serrano survived one of the toughest tests of her career Saturday night to make history and secure a huge payday for her next fight.

One of best female fighters in boxing history got off to a very slow start against a relentless Erika Cruz, who demonstrated remarkable toughness as she fought through a grotesque gash on her forehead for six-plus rounds. Cruz’s constant aggression made matters difficult for Brooklyn’s Serrano, but Serrano still won their 10-round, 126-pound title unification bout by unanimous decision at Madison Square Garden’s Hulu Theater.

Judges Steve Weisfeld (98-92) and Frank Lombardi (98-92) scored eight rounds apiece for Serrano, who won seven rounds according to judge Mark Constantino (97-93).

The 34-year-old Serrano (44-2-1, 30 KOs), who has won world titles in seven divisions, became a fully unified champion for the first time. She successfully defended her IBF, IBO, WBC and WBO featherweight titles and won the WBA 126-pound championship from Cruz.

Serrano secured her place in her rematch with Katie Taylor as well. Promoter Eddie Hearn announced in the ring after Serrano’s win that Ireland’s Taylor (22-0, 6 KOs) and Serrano will fight again for Taylor’s four lightweight titles May 20 in Dublin.

In addition to losing her WBA featherweight title, Cruz’s 14-fight winning streak came to an end. Prior to Saturday night, Cruz (15-2, 3 KOs) hadn’t lost since fellow Mexican Alondra Gonzalez beat her by majority decision in Cruz’s second pro fight, which took place in June 2016 in Cholulua, Mexico.

“She’s a Mexican champion,” Serrano told DAZN’s Chris Mannix in the ring. “And we knew that, and I said it from the very beginning. I knew she wasn’t just gonna let me take her belt. She worked hard for this, I worked hard for mine. And that’s what we expected, that’s what we trained for. But I wouldn’t expect any less than that.”

Serrano added that she was happy that their highly competitive fight went past the fourth round because Cruz’s cut was so bad. Had the bout been stopped prior to the end of the fourth round, it would’ve been declared a no-contest because it was caused by an accidental head-butt in the third round.

Cruz fell to the canvas 30 seconds into the 10th round, but Gonzalez rightly ruled that their legs got tangled. Once Cruz got up, she went right back to pursuing Serrano, who as many punches as she could to fend off the relentless ex-champion.

Serrano’s strength was the difference during the eighth and ninth rounds. Cruz continued to press forward, even though blood poured from the cut on her forehead, but she took a lot of flush punches in those two rounds.

Serrano spent much of the seventh round landing hard lefts and rights on the resilient Cruz, whose grotesque gash bled worse than ever by the end of that round.

A left-right-left-right-left combination by Serrano rocked Cruz with about 50 seconds to go in the sixth round. Cruz stumbled backward, into the ropes, and Serrano went for the knockout.

Cruz incredibly withstood Serrano’s barrage of punches and made it to the end of the sixth round.

Cruz continued to land flush punches on Serrano in the fifth round, even as she bled badly from the cut on her forehead. Serrano sat down on her punches in the final 40 seconds of the fourth round, though, and temporarily kept the relentless Cruz from throwing punches.

An accidental clash of heads caused a nasty cut across Cruz’s forehead with about 30 seconds to go in the third round. Blood poured from Cruz’s cut for the remainder of the round, but Serrano escaped that collision unscathed.

A right-left combination by Cruz caught Serrano barely 30 seconds into the second round. Cruz didn’t have enough power to affect Serrano, but she landed more punches during the second round, just as she did during the opening two minutes.

Cruz connected several times in the opening round, when she was the busier fighter.

Keith Idec is a senior writer/columnist for BoxingScene.com. He can be reached on Twitter @Idecboxing.