LAS VEGAS — Just when it seemed Tim Tszyu was mounting a comeback against Sebastian Fundora, the former WBO junior middleweight champion decided he couldn’t continue in their 12-round rematch Saturday night.

Tszyu declined to answer the bell for the eighth round, which gave Fundora the decisive victory he sought in their second 154-pound title fight. Referee Harvey Dock declared Fundora the winner by technical knockout after an action-packed seventh round on the Manny Pacquiao-Mario Barrios undercard at MGM Grand Garden Arena.

Tszyu admitted during his post-fight interview with Jim Gray that he didn’t have the will to continue fending off the 6-foot-6 Fundora, who used his height and reach better than he did during many of his previous fights. Fundora (23-1-1, 15 KOs), a southpaw from Coachella, California, retained his WBC super welterweight title.

“He’s one tough [expletive],” Tszyu said. “And I tried to give it everything, but I just couldn’t do it and the victory belonged to Sebastian Fundora, the best 154-pounder in the world. … He was just a better man. [It] was just hard to land. He’s tall as [expletive]. At times I felt like I was shadowboxing with myself.”

The 5-foot-9 Tszyu (25-3, 18 KOs) failed to avenge his first professional loss 15½ months after Fundora defeated him by split decision at nearby T-Mobile Arena. Tszyu lost that bloody battle in part because he suffered a gruesome cut at the center of his hairline late in the second round.

Tszyu, 30, stopped American Joey Spencer (19-2, 11 KOs) in the fourth round in April, but he now has lost two of his past three fights by TKO.

Bakhram Murtazaliev, the unbeaten IBF junior middleweight champion, brutalized Tszyu in the Sydney, Austraila, native’s following fight. Tszyu was knocked down four times and was stopped in the third round of that Oct. 19 bout in Orlando, Florida.

Fundora dropped Tszyu in the first round Saturday night, yet he slowly but surely started to land right hands in return.

Tszyu’s left hook caught Fundora with a little more than two minutes remaining in the seventh round. Tszyu then blasted Fundora with a right hand that drew a loud response from the crowd.

They traded power shots during the fan-friendly remainder of an action-packed seventh. A demoralized Tszyu placed both of his arms on the ropes after the bell sounded to end the round, though, and told his cornermen several seconds later that he didn’t want to fight anymore.

Tszyu’s right hand drilled Fundora with just over two minutes on the clock in the fifth round. The challenger seemed more comfortable by then and took Fundora’s punches better.

Fundora trapped Tszyu in a corner about a minute into the fourth round and fired hard shots at his head. Tszyu blocked and slipped some of those punches, and later landed a left hook that made Fundora reset his feet.

After getting dropped in the first round and cut in the second, Tszyu came out swinging at the start of the third. None of those right hands hit his target, though it temporarily kept Fundora from coming forward.

A straight left by Fundora knocked Tszyu to the seat of his trunks with 1:16 remaining in the first round. That knockdown represented a change in tone from the opening round of their first fight, in which Tszyu regularly landed right hands on Fundora.