LOS ANGELES – Shawn Porter proved Saturday night that the odds-makers and countless others had no idea what was about to happen.

Dismissed by many as a huge underdog who couldn’t compete with Errol Spence Jr., Porter pressed the unbeaten Spence into the most difficult fight of Spence’s career. It was Spence, however, who knocked down Porter late in the 11th round and won their 12-round welterweight title unification bout by split decision at Staples Center.

Two judges – California’s Rey Danesco (116-111) and New Jersey’s Steve Weisfeld (116-111) – scored their close, physical fight for Spence. One judge – New Jersey’s Larry Hazzard Jr. – credited Porter with a victory (115-112) in what was an action-packed, back-and-forth fight that brought most fans among the crowd of 16,702 to their feet after their FOX Sports Pay-Per-View main event ended.

Spence (26-0, 21 KOs), who was a 10-1 favorite, defended his IBF welterweight title for the fourth time and won the WBC 147-pound crown from Porter (30-3-1, 17 KOs). The DeSoto, Texas, native also set up a potential showdown with 40-year-old Filipino legend Manny Pacquiao by winning what will be a “Fight of the Year” candidate.

Opposing Pacquiao, the WBA champ, would afford Spence the chance to add a third 147-pound crown and set up the fight virtually every boxing fan wants to see – Spence versus WBO champion Terence Crawford.

Spence also could fight former WBC champion Danny Garcia next. Garcia stood side by side with Spence as he was interviewed following his win.

Ken Porter, Shawn’s father/trainer, said during the week he wanted to see how Spence would respond during a dogfight because he didn’t think Spence had been in one as a pro. Las Vegas’ Porter made it exactly that type of fight very early in it, bullied the strong southpaw backward at times and tested Spence’s will like he hadn’t experienced in any of his first 25 professional fights.

“Shawn Porter is a rough and awkward fighter,” Spence told FOX’s Heidi Androl after his victory. “I didn’t get off what I wanted to. He’s a true champion. He made it tough.”

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CompuBox counted 49 more overall punches for Spence (221-of-745 to 172-of-744). According to CompuBox, Spence landed more power punches (184-of-416 to 142-of-553) and jabs (37-of-329 to 30-of-191).

The 31-year-old Porter still lost the third close decision of his career, each in welterweight title fights. Before the 29-year-old Spence edged him Saturday night, he had lost only a majority decision to Kell Brook and a unanimous decision to Keith Thurman.

Like Spence, Brook and Thurman were undefeated when they topped Porter.

The former WBC and IBF champion handled another close loss with complete class.

“He’s a strong kid,” Porter said. “We both came in to do the job. I think I had a little more than he expected, but he handled it. Congratulations to him and his team. We’re proud of what we did.”

After dropping Porter with a left hand late in the 11th round, Spence aggressively went for the knockout at the start of the 12th round. Porter valiantly went toe-to-toe with the strong southpaw and drilled Spence with a right hand that snapped Spence’s head back just before the midway mark of the final round.

The champions hammered away at one another for the rest of that 12th round, but neither fighter landed the type of shot that hurt his opponent.

Porter’s right cross caught Spence early in the 11th round, as Spence was backed up to the ropes. A perfect left hook Spence clipped Porter perfectly, buckled his legs and made his left glove touch the canvas when there was exactly one minute to go in the 11th.

Porters seemed surprised that he went down. He popped up quickly and, rather than holding Spence to survive the round, went after Spence and pushed him backward.

A right hand by Porter and Spence’s left landed just before the end of the 10th round. Both boxers appeared to land punches after the bell to end the 10th – first Porter, then Spence.

A right hand by Porter snapped Spence’s head back a little more than a minute into the ninth round. Spence and Porter traded power shots from close distances during a thoroughly entertaining exchange in the final minute of the ninth.

Porter’s pressure put Spence on his back foot at the start of the eighth round, but he couldn’t land an impactful punch. Eventually, however, an overhand right by Porter cracked Spence with about 40 seconds to go in the eighth, just after Spence missed a left hand.

Porter landed a solid right hand to the body just before the halfway point of the seventh round. They spent much of the rest of the round pushing for position, but Porter snuck in another hard right to the body just before the seventh round ended.

Spence pushed Porter into the ropes and tried to attack his body early in the sixth round. Porter covered up well, spun out of that position and immediately backed Spence into the ropes.

They were in the center of the ring soon thereafter, hammering away at each other’s body. Spence landed a straight left hand to Porter’s head several seconds later.

Porter clipped Spence with a straight right early in the fifth round. Porter took a straight left from Spence well late in the fifth round and kept coming forward.

Porter caught Spence with a left hook that snucked around Spence’s guard early in the fourth round. Another left hook by Porter caught Spence in an exchange later in the fourth.

Spence came back to land an overhand left later in the fourth that moved Porter backward.

About a minute into the third round, Reiss warned Porter for holding Spence behind his head. The action intensified later in the third, when Porter pushed Spence backward.

Porter landed a hard, overhand right to the side of Spence’s head. Spence landed to Porter’s body, but Reiss warned Spence for a low blow during that sequence, which Spence acknowledged by extending his glove to Porter.

Porter attempted to bully Spence into a neutral corner late in the second round. Spence responded by slipping away and landing two hard body shots.

Porter previously snuck in a hard jab during an exchange.

The crowd got excited just before the halfway point of the second round because Spence stumbled backward, across the ring. Spence tripped, but the crowd seemed to think Porter hurt him with a punch.

A short, straight left hand by Spence caught Porter as he came forward early in the second round.

Porter twice tried to push Spence back, into the ropes, in the first half of the first round. Spence held his ground to show Porter he wouldn’t be moved around easily.

With just over 30 seconds to go in the opening round, Spence caught Porter with a short right hook as Porter got into a squatting position while trying to avoid Spence’s shots.

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