A crisp counter left uppercut from Terence Crawford dropped Shawn Porter early in the 10th round of their World Boxing Organization title fight in Las Vegas on Saturday night. Moments later, a concussive right hook to Porter’s temple started a sequence that ended with Porter on the canvas and his trainer and father, Kenny Porter, signaling to the referee to stop the fight.

The technical knockout brought a definitive end to a tense, rugged, dramatic bout between top-tier welterweights. Crawford, now 38-0, is widely acknowledged as one of the division’s top two fighters, alongside Errol Spence Jr. By defeating Porter, Crawford defended his belt and earned a signature win over an elite performer. Entering the final phase of the fight, Porter, now 31-4-1, still had a chance to win a decision, but Crawford erased it with a stunning display of power.

But then came a plot twist.

Crawford, 34, has long wanted to fight Spence, but boxing business has prevented it from happening. Premier Boxing Champions manages Spence, while Crawford is signed to Top Rank. The two outfits rarely cooperate, but Crawford’s title defense against Porter, a P.B.C. client, seemed to signal hope that the two sides would collaborate on future fights.

Yet after the fight, Crawford, who is now a free agent, according to a person who wasn’t authorized to discuss his contract publicly, spoke of his relationship with Top Rank and its president, Bob Arum, in the past tense, hinting at a shift in the balance of promotional power.

“Bob couldn’t secure the Spence fight when I was with him,” Crawford said at the postfight news conference with a silent Arum beside him. “How can he secure it when I’m not with him?”

Porter, a 34-year-old former champion, won the first round on all three scorecards by pressuring Crawford. The fight began in earnest in round No. 2, when Porter landed an overhand right to Crawford’s face, drawing Crawford into the first of several exchanges.

By Round 7, Porter’s braids, tightly bound in cornrows, were fraying. He landed another overhand right. Crawford answered with a left hand to the face, and finished the round by landing a jab and a left.

In the eighth round, Crawford’s breathing grew heavier. He unleashed long punches to keep Porter at a distance. Porter landed a series of rights anyway.

Crawford closed the ninth round with a thudding left uppercut, and in the 10th he landed a similar punch to score the first knockdown. When Porter went down a second time, his father stopped the fight. The elder Porter, who oversaw Shawn Porter’s training, hinted that his son hadn’t prepared properly for Crawford but still gave him credit.

“All things combined, he can do it,” Kenny Porter said of Crawford. “Offense, defense, switching sides. Speed, quickness and power.”

According to CompuBox, Crawford landed 98 of 328 punches, while Porter connected on 79 of 347. While Porter connected on two more power punches, Crawford’s statistical edge came mainly from his jab — he landed 33, compared with Porter’s 12. Entering Round 10, Crawford led, 86-85, on two official scorecards and 87-84 on the third.

Porter, whose four career losses have come in competitive bouts against welterweight champions, announced his retirement after the loss to Crawford.

“I’ve given this sport a great deal,” Porter said. “After you’ve fought everybody at the top, what else do you do? I don’t want to be a gatekeeper.”

And Crawford, who has won nine straight bouts by knockout, said it was too soon to choose from among his options.

“We’re going to go to the drawing board and see what’s best for Terence Crawford,” he said.

He didn’t sound inclined to re-sign with Top Rank, but it’s possible.

Crawford could continue as a free agent, leveraging his status and undefeated record to score one-off fights across boxing’s promotional landscape. Saúl Álvarez has used that formula to become the undisputed super-middleweight champion, winning belts in fights with the England-based promoter Matchroom Boxing, then jumping to P.B.C. to defeat Caleb Plant and unify the title.

Or Crawford could join P.B.C., which manages Spence, who holds the World Boxing Council and International Boxing Federation titles, and Yordenis Ugas, the World Boxing Association champion.

“You already know who I want,” Crawford said after the fight.

He meant Spence, the undefeated fighter from Dallas, who recovered from a fiery car crash in 2019 to defeat Danny Garcia last year, and who underwent surgery in August to repair a torn retina. Spence watched the fight from ringside, and, moments after the referee declared Crawford the winner, was pictured shaking his head and leaving the arena.

The P.B.C. on Fox Instagram account posted a photo illustration of Spence and Crawford squaring off in the ring. Spence later shared the post to his Instagram Stories feed.