LOS ANGELES -- Errol Spence Jr. and Shawn Porter waged the possible fight of the year, but it was Spence who got the nod in an action-packed victory by split decision to unify two welterweight world titles Saturday night at Staples Center.

After a little bit of a slow start, the fight turned into an all-out slugfest in the third round and never let up as the crowd of 16,702 spent long portions of the fight on its feet. But it was Spence -- who scored a knockdown in the 11th round -- winning 116-111 on two scorecards, while one judge favored Porter 115-112. ESPN had it 115-112 for Spence.

Both men said ahead of the fight that the winner should be viewed as the best fighter in the 147-pound division. It is a talent-rich weight class that is also home to Manny Pacquiao, the all-time great legend who owns one of the belts, as well as Terence Crawford, who owns the other major belt and ranks No. 2 in the ESPN pound-for-pound best poll.

But Spence, who was No. 5 on the pound-for-pound list heading into the fight, made his case to rise, as he scored the biggest win of his career against a two-time world titlist in a tremendous battle.

"It feels good to win. This is a lifetime dream. It shows hard work pays off," Spence said of unifying belts. "Thanks, Shawn Porter, my whole team and all my Texas people for coming out. Shawn Porter is a rough and awkward fighter. I didn't get off what I wanted to. He's a true champion. He made it tough."

Spence retained his title for the fourth time and accomplished his long-stated goal of unifying belts, even if he did not get the knockout he so vociferously insisted he would notch.

It was an impressive follow-up performance for Spence, who shut out four-division world titlist Mikey Garcia in March in a high-profile decision victory. Garcia was a much smaller man who had called Spence out and moved up two weight divisions to challenge him. Spence never imposed himself. Instead, he easily outboxed Garcia in a one-sided rout.

He promised things would be different against Porter, a bona fide welterweight with a strong résumé, loads of experience and a good chin. Spence was true to his word, delivering everything he said he would other than a knockout.

"All my punches have bad intentions. Boxing Mikey Garcia, I wanted to show people I could do it with that style," Spence said. "Porter was throwing a lot. I wanted to show I was the bigger and stronger welterweight."

The fight began with each man doing what most expected, with Spence (26-0, 21 KOs), 29, a southpaw from DeSoto, Texas, working his jab and Porter looking to force his way inside.

The fight turned into a slugfest in the third round as they attacked each other with abandon. Spence landed a hard counter left on the button, but Porter (30-3-1, 17 KOs), 31, of Las Vegas, took it well, and as they continued to trade, he got in a big right hand.

Porter, making his second title defense, opened the fourth round with a flurry of punches that all appeared to land and then connected with a right hand. Spence came back with a counter left, and they battled back and forth. Porter landed a tremendous right hand on the top of Spence's head, and then a left from Spence rocked Porter's head back with 30 seconds to go in an action-packed round.

They continued to battle at a fast pace in the middle rounds, with Porter forcing everything he could to stay on Spence on the inside, and Spence looking to land his left hand.

Porter had a strong seventh round. He started fast and never slowed down as he outworked and outhustled Spence, who might have been looking for a breather after such an intense pace through the first half of the bout.

Porter landed a left hand to the head that wobbled Porter about 30 seconds into the eighth round and continued to fire away. He landed a right hand late in the round in which Spence did not seem to do much.

The ninth round also was action-packed with both fighters landing powerful shots but Porter, who would not given an inch, seeming to land the more telling right hands to the head over and over. He snapped Spence's head back with one early in the round, and they fought toe-to-toe for most of the round.

As the 11th round opened, the crowd broke into a "Porter! Porter!" chant as the fighters waded back into battle in another round of fierce back-and-forth action..

Spence scored the definitive moment of the fight late in the 11th round when he cracked Porter with a left hand on the chin to score a knockdown. Porter easily beat the count and went after Spence with abandon and incredible determination.

"I think that knockdown was the difference. I couldn't come back to the corner with my head down after that," Porter said.

Both fighters were going for a knockout in the 12th in yet another wildly exciting round. Neither man would give in as they traded to the final bell with the crowd on its feet.

Spence landed 221 of 745 punches (30%), and Porter landed 172 of 744 (23%), the most Spence has been hit in his career.

Porter was proud of his effort but credited Spence, whom he has known and been friendly with for years.

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

Porter had never shied away from a challenge and embraced fighting Spence when few other top welterweights were interested. Porter had already defeated the likes of Danny Garcia for a vacant world title last September, formidable mandatory challenger Yordenis Ugas in March as well as Adrien Broner, Andre Berto, Devon Alexander and Paulie Malignaggi.

Though he met his match in Spence, he knew he had been in a special fight.

"I've said this before: I've had a lot of experience in the boxing ring," Porter said. "Did you enjoy that fight?"

The answer from all probably was a resounding yes, with Spence enjoying it as much as anyone. After the fight, Garcia, a former welterweight and junior welterweight titlist, who was part of the Fox PPV broadcast team, was in the ring. Spence was asked if he would fight him -- even though he has said he wants Pacquiao or Crawford to further unify the division.

Said Spence: "I've told my team: You line them up, I'll knock them down."